fix(sources): address Copilot review on #1014

Five findings from the automated review, fixed with targeted tests where
behavior changed:

1. Transformation Protocol (transforms.py). The registry mixed a bytes-to-str
   transform (utf8_replace_invalid) with str-to-str transforms under a single
   Callable[..., str] type, misleading static type checkers and adapter
   authors. Introduced a Transformation Protocol with __call__(data: bytes|str)
   -> str and retyped the registry + get_transformation return.

2. Drawer-id collision risk (context.py). Switched _build_drawer_id from
   sha1[:16]=64 bits to sha256[:24]=96 bits. 64 bits sits uncomfortably
   close to the birthday bound for palace-sized corpora; 96 bits keeps the
   collision probability negligible while preserving the existing
   <prefix>_<chunk> layout adapters rely on.

3. Fresh-schema KG columns (knowledge_graph.py). source_drawer_id and
   adapter_name now live in the canonical CREATE TABLE so new palaces don't
   take an ALTER round-trip on first open. _migrate_schema stays for legacy
   palaces (SQLite has no ADD COLUMN IF NOT EXISTS, so PRAGMA introspection
   is still needed there).

4. Identity-shim comment (transforms.py). Comment said the adapter-specific
   transforms "raise if invoked without adapter context" but they return
   the input unchanged. Updated the comment to match the actual identity-
   shim behavior Copilot suggested.

5. Test docstring (test_sources.py). Comment mentioned default_factory=list
   but SourceRef.options uses default_factory=dict. Corrected.

Tests: 1020 passed (up from 1018), +2 new tests for the sha256 id shape
and the fresh-schema column presence on new palaces.

mempalace/knowledge_graph.py

@@ -83,6 +83,8 @@ class KnowledgeGraph:
                 confidence REAL DEFAULT 1.0,
                 source_closet TEXT,
                 source_file TEXT,
+                source_drawer_id TEXT,
+                adapter_name TEXT,
                 extracted_at TEXT DEFAULT CURRENT_TIMESTAMP,
                 FOREIGN KEY (subject) REFERENCES entities(id),
                 FOREIGN KEY (object) REFERENCES entities(id)
@@ -99,11 +101,12 @@ class KnowledgeGraph:
     def _migrate_schema(self, conn):
         """Backwards-compatible schema migration for older triples tables.
 
-        RFC 002 §5.5 adds two optional provenance columns so adapter-written
-        triples can be traced back to (a) the specific drawer that produced
-        them and (b) the adapter that authored them. Older palaces predate
-        these columns; SQLite has no ``ADD COLUMN IF NOT EXISTS``, so we
-        introspect the schema first and only issue the ALTER if needed.
+        Fresh palaces get ``source_drawer_id`` / ``adapter_name`` (RFC 002 §5.5)
+        directly from the canonical ``CREATE TABLE`` above, so this path is a
+        no-op on new installs. It exists for palaces that were created before
+        those columns were added: SQLite has no ``ADD COLUMN IF NOT EXISTS``,
+        so we introspect the schema and only issue the ALTER when the column
+        is missing.
         """
         existing = {row["name"] for row in conn.execute("PRAGMA table_info(triples)")}
         if "source_drawer_id" not in existing:

mempalace/sources/context.py

@@ -126,15 +126,17 @@ class PalaceContext:
 
 
 def _build_drawer_id(record: DrawerRecord) -> str:
-    """Deterministic drawer id: ``<sha1(source_file)>_<chunk_index>``.
+    """Deterministic drawer id: ``<sha256(source_file)[:24]>_<chunk_index>``.
 
     Matches the shape existing miners rely on (``source_file`` + chunk index
     pair) while keeping the id chroma-safe (no separators that collide with
-    existing metadata values). Adapters that need a different id scheme can
-    bypass :meth:`PalaceContext.upsert_drawer` and write through
-    ``drawer_collection.upsert`` directly.
+    existing metadata values). 96-bit SHA-256 prefix keeps collision risk
+    negligible across corpora the size of a palace (sha1@64 bits was too
+    close to the birthday bound for large ingests). Adapters that need a
+    different id scheme can bypass :meth:`PalaceContext.upsert_drawer` and
+    write through ``drawer_collection.upsert`` directly.
     """
     import hashlib
 
-    digest = hashlib.sha1(record.source_file.encode("utf-8")).hexdigest()[:16]
+    digest = hashlib.sha256(record.source_file.encode("utf-8")).hexdigest()[:24]
     return f"{digest}_{record.chunk_index}"

mempalace/sources/transforms.py

@@ -24,7 +24,19 @@ conformance suite can locate and apply them.
 from __future__ import annotations
 
 import re
-from typing import Callable
+from typing import Protocol, Union
+
+
+class Transformation(Protocol):
+    """Callable signature every reserved transformation conforms to.
+
+    Accepts the current stage of the pipeline — ``bytes`` on input
+    (``utf8_replace_invalid``) or ``str`` after decoding — and returns ``str``.
+    Adapters compose them in declaration order; the first step operates on the
+    original source bytes, every subsequent step on the prior step's output.
+    """
+
+    def __call__(self, data: Union[bytes, str], /) -> str: ...
 
 
 # ---------------------------------------------------------------------------
@@ -90,12 +102,14 @@ def blank_line_drop(text: str) -> str:
 
 # The following reserved transformations are declared in the spec but are
 # deeply adapter-specific. Rather than guess a single reference implementation
-# now, we provide identity shims that raise if invoked without adapter-supplied
-# context. Adapters that declare these MUST either override with a concrete
-# implementation or provide a namespaced reference under
+# now, we provide identity shims that leave the input unchanged when no
+# adapter-specific implementation is available. Adapters that declare these
+# MUST either override with a concrete implementation or provide a namespaced
+# reference under
 # ``mempalace.sources.transforms.<adapter_name>_<transform_name>`` (per the
 # module docstring). The conformance suite looks up the adapter-specific
-# implementation first, falling back to these only when none exists.
+# implementation first, falling back to these identity shims only when none
+# exists.
 
 
 def strip_tool_chrome(text: str) -> str:
@@ -146,7 +160,10 @@ def speaker_role_assignment(text: str) -> str:
 
 # Reserved transformation name → reference implementation.
 # Adapters look up by name to compose a round-trip pipeline during testing.
-RESERVED_TRANSFORMATIONS: dict[str, Callable[..., str]] = {
+# The value conforms to the :class:`Transformation` protocol above; we type
+# it as that Protocol rather than a concrete ``Callable`` so static checkers
+# accept both the bytes→str (``utf8_replace_invalid``) and str→str shapes.
+RESERVED_TRANSFORMATIONS: dict[str, Transformation] = {
     "utf8_replace_invalid": utf8_replace_invalid,
     "newline_normalize": newline_normalize,
     "whitespace_trim": whitespace_trim,
@@ -163,7 +180,7 @@ RESERVED_TRANSFORMATIONS: dict[str, Callable[..., str]] = {
 }
 
 
-def get_transformation(name: str) -> Callable[..., str]:
+def get_transformation(name: str) -> Transformation:
     """Resolve a reserved transformation by name.
 
     Raises :class:`KeyError` if the name is neither reserved nor registered as

tests/test_sources.py

@@ -103,7 +103,7 @@ def test_source_summary_default_uses_adapter_name():
 
 
 def test_source_ref_options_default_is_empty_dict():
-    # Frozen dataclass must not share default_factory=list instance across instances.
+    # Frozen dataclass must not share a default_factory=dict instance across instances.
     a = SourceRef(uri="a")
     b = SourceRef(uri="b")
     a.options["touched"] = True
@@ -280,6 +280,27 @@ def test_palace_context_upsert_drawer_stamps_adapter_metadata():
     assert meta["chunk_index"] == 2
 
 
+def test_palace_context_drawer_id_is_sha256_prefix_not_sha1():
+    """Guards against the pre-review sha1[:16]=64-bit id scheme.
+
+    64-bit ids sit close to the birthday bound for palace-sized corpora.
+    The helper uses sha256[:24]=96 bits so collision risk stays negligible.
+    """
+    import hashlib
+
+    from mempalace.sources.context import _build_drawer_id
+
+    src = "/an/absolute/path/to/a/file.txt"
+    record = DrawerRecord(content="x", source_file=src, chunk_index=3)
+    drawer_id = _build_drawer_id(record)
+
+    expected_prefix = hashlib.sha256(src.encode("utf-8")).hexdigest()[:24]
+    assert drawer_id == f"{expected_prefix}_3"
+    # Negative: the old sha1 scheme MUST NOT produce the same id.
+    sha1_prefix = hashlib.sha1(src.encode("utf-8")).hexdigest()[:16]
+    assert drawer_id != f"{sha1_prefix}_3"
+
+
 def test_palace_context_skip_current_item_sets_flag():
     ctx = PalaceContext(
         drawer_collection=_FakeCollection(),
@@ -355,6 +376,25 @@ def test_knowledge_graph_add_triple_accepts_source_drawer_id_and_adapter_name(tm
         kg.close()
 
 
+def test_knowledge_graph_fresh_schema_includes_new_columns(tmp_path):
+    """Brand-new palaces should get source_drawer_id / adapter_name directly
+    from CREATE TABLE, not via a post-hoc ALTER. _migrate_schema exists only
+    for legacy palaces."""
+    import sqlite3
+
+    from mempalace.knowledge_graph import KnowledgeGraph
+
+    kg = KnowledgeGraph(db_path=str(tmp_path / "fresh.sqlite3"))
+    try:
+        conn = sqlite3.connect(str(tmp_path / "fresh.sqlite3"))
+        cols = {row[1] for row in conn.execute("PRAGMA table_info(triples)")}
+        conn.close()
+        assert "source_drawer_id" in cols
+        assert "adapter_name" in cols
+    finally:
+        kg.close()
+
+
 def test_knowledge_graph_migration_adds_missing_columns_to_old_schema(tmp_path):
     """An old-schema triples table (pre-RFC 002) should auto-migrate on open."""
     import sqlite3