feat(privacy): treat Tailscale CGNAT range (100.64.0.0/10) as local

2 files changed, 60 insertions, 0 deletions. 2 new tests (RED-first).

Follow-up to #1224's privacy warning. The URL-based heuristic in
``mempalace.llm_client._endpoint_is_local`` shipped without recognizing
Tailscale's CGNAT range (100.64.0.0/10), so a user running LM Studio,
Ollama, or any local LLM accessible via a Tailscale-assigned 100.x.x.x
address would currently get a wrong privacy warning — Tailscale
addresses are network-private (only reachable inside the user's
Tailnet) but they're not RFC1918, so the heuristic was treating them
as external.

This PR adds CGNAT recognition: when the hostname starts with ``100.``
AND the second octet is between 64 and 127 inclusive, it's classified
as local. Addresses in 100.x.x.x outside that range (i.e. second octet
< 64 or > 127) are regular allocated public space and remain external,
so a user pointing at a public 100.0.0.1 still gets the warning.

Concrete user impact:

  Before: ``mempalace init --llm-provider openai-compat --llm-endpoint http://100.100.50.50:1234``
          (LM Studio on Tailnet) → triggers privacy warning incorrectly.

  After:  same command → no warning. data stays inside the user's
          Tailnet, which is what the warning is supposed to protect against.

TDD: 2 tests added in ``tests/test_llm_client.py``, both RED-first.

1. ``test_openai_compat_provider_tailscale_cgnat_endpoint_is_local``
   — covers three Tailscale CGNAT addresses (start, middle, near-end
   of the range) and pins they're all classified local. This was the
   RED that drove the implementation.

2. ``test_openai_compat_provider_outside_tailscale_cgnat_is_external``
   — pins the boundary on both sides: addresses with second octet 0-63
   and 128-255 stay external. Prevents future "treat all 100.x.x.x as
   local" overcorrection.

Tests: 1388 total mempalace tests pass. 2 pre-existing environmental
failures unrelated to this change (chromadb optional dep). Ruff check
+ format both clean.

Backwards compatible: only widens the local-recognition set. Anything
classified local before is still classified local; anything classified
external before remains so unless it's specifically in the CGNAT range.

Out of scope (tracked for future iteration based on real user feedback,
not built speculatively): pre-init confirmation prompt before sending
to external API, persistent ``private-only`` config flag that refuses
external endpoints entirely, explicit cloud-provider name detection
("Using Anthropic's hosted API at ..." vs the current generic warning).

tests/test_llm_client.py

@@ -378,3 +378,51 @@ def test_anthropic_provider_default_endpoint_is_external():
         f"Default AnthropicProvider endpoint must be external; got "
         f"is_external_service={p.is_external_service} for endpoint={p.endpoint}"
     )
+
+
+# ── Tailscale CGNAT range (issue #25 follow-up to #24) ──────────────────
+#
+# Tailscale assigns addresses in 100.64.0.0/10 (CGNAT range): first octet
+# always 100, second octet 64-127 inclusive. Users running LM Studio /
+# Ollama / any local LLM accessible via Tailscale would currently
+# (post-#24, pre-#25) get a wrong privacy warning because the heuristic
+# doesn't recognize CGNAT as private. These tests pin the fix.
+
+
+def test_openai_compat_provider_tailscale_cgnat_endpoint_is_local():
+    """Tailscale CGNAT range (100.64.0.0/10) — IPs where the first octet
+    is 100 AND the second octet is 64-127 inclusive — must be classified
+    as local. Tailscale users running LM Studio on their Tailnet should
+    not trigger the external-API warning.
+    """
+    cases = [
+        ("http://100.64.0.1:1234", "start of CGNAT"),
+        ("http://100.100.50.50:1234", "middle of CGNAT (typical Tailscale assignment)"),
+        ("http://100.127.255.254:1234", "near end of CGNAT"),
+    ]
+    for endpoint, label in cases:
+        p = OpenAICompatProvider(model="any", endpoint=endpoint)
+        assert p.is_external_service is False, (
+            f"Tailscale CGNAT address {endpoint} ({label}) must be classified "
+            f"local; got is_external_service={p.is_external_service}"
+        )
+
+
+def test_openai_compat_provider_outside_tailscale_cgnat_is_external():
+    """Addresses in 100.x.x.x that fall OUTSIDE the CGNAT range
+    (100.64.0.0 - 100.127.255.255) are public IPs in regular allocated
+    space and must remain classified as external. Specifically: anything
+    where the second octet is < 64 or > 127.
+    """
+    cases = [
+        ("http://100.0.0.1:1234", "below CGNAT (public)"),
+        ("http://100.63.255.255:1234", "just below CGNAT (boundary)"),
+        ("http://100.128.0.0:1234", "just above CGNAT (boundary)"),
+        ("http://100.255.255.255:1234", "well above CGNAT"),
+    ]
+    for endpoint, label in cases:
+        p = OpenAICompatProvider(model="any", endpoint=endpoint)
+        assert p.is_external_service is True, (
+            f"Address {endpoint} ({label}) is OUTSIDE Tailscale CGNAT and "
+            f"should remain external; got is_external_service={p.is_external_service}"
+        )