[bug] db/features.py feature-cache keyed by id() — collision hazard after GC

[100yenadmin]

Found by: the PR #149 fix-forward (a new test perturbed a stale-cache hit into a non-deterministic ubuntu-py3.13 CI failure).

db/features.py's feature-probe cache is keyed by id() of the connection object. Python reuses id() values after an object is garbage-collected — so a new connection can collide with a GC'd one's cache entry and read stale feature-probe results (e.g. "sqlite-vec available" / FTS5 tokenizer support) for the wrong connection. This is non-deterministic and surfaced as a flaky test in #149 (worked around there by clearing the cache in the test db fixture — a band-aid, not a fix).

Fix: key the cache on something stable for the connection's lifetime (e.g. the resolved DB path + role, or a per-connection attribute set at open time) instead of id(). Audit for the same id()-keying pattern elsewhere.

Target: v0.2.0.