chore(release): bump version strings to 3.3.0 and curate CHANGELOG

Prepare develop for the 3.3.0 release cycle.

Version bumps:
- mempalace/version.py: 3.2.0 -> 3.3.0
- pyproject.toml: 3.2.0 -> 3.3.0
- README.md: pyproject.toml label and shields.io badge
- uv.lock: mempalace 3.0.0 -> 3.3.0 (also fills in resolved dev/extras)

CHANGELOG.md:
- Close out the stale [Unreleased] section as [3.2.0] - 2026-04-12
  (v3.2.0 was tagged on that date but the release flip was never made)
- Add a fresh [Unreleased] - v3.3.0 section covering the 49 commits
  since v3.2.0: closet layer, BM25 hybrid search, entity metadata,
  diary ingest, cross-wing tunnels, drawer-grep, offline fact checker,
  LLM-based closet regen, hall detection, cosine-distance fix,
  multi-agent locking, README audit, etc.
- Adopt Keep a Changelog + SemVer framing
- Add version compare reference links at the bottom
- Fix stale milla-jovovich/mempalace preamble URL to MemPalace/mempalace

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "mempalace"
-version = "3.2.0"
+version = "3.3.0"
 description = "Give your AI a memory — mine projects and conversations into a searchable palace. No API key required."
 readme = "README.md"
 requires-python = ">=3.9"