Storage pools key on non-canonicalized PathBuf — two spellings of one file produce two Database::create calls and a redb lock error

Defect

Both redb connection pools — ruvector-core/src/storage.rs and ruvector-graph/src/storage.rs (the latter now on the Weak+Drop shape after #907) — key their pool map on a PathBuf that is absolutized (joined onto cwd when relative) but never canonicalized. Two spellings of the same file resolve to two slots:

• a/b/../b/x.db vs a/b/x.db
• a symlinked directory vs its target
• on macOS, differing case on a case-insensitive filesystem

Each slot independently calls Database::create against the same underlying file; the second open fails with redb's "Database already open. Cannot acquire lock" — the exact error the pool exists to prevent.

This was noted in #907's tail ("Related, pre-existing in both pools") and is split out here so it survives #907 closing. It predates #902 in ruvector-core as well.

Suggested fix

Canonicalize before keying, in both pools:

1. The parent directory always exists by the time the key is formed (both pools create_dir_all it first), so path.parent().canonicalize()?.join(file_name) is total for the create-if-absent case — Path::canonicalize on the full path fails when the DB file doesn't exist yet, so canonicalize the parent, not the file.
2. Keep the traversal check on the user-supplied path (it exists to reject intent, and canonicalizing first would erase the evidence), and use the canonical form only as the pool key + Database::create argument.
3. Regression tests, same in both crates: open via dir/x.db and dir/sub/../x.db concurrently — must share one Database (no lock error, both handles see each other's writes); open via a symlinked dir and the real dir — same assertion.

Since the two pools are intentionally copies of one reference shape (ADR-340 invariant 3), fix them in the same PR so they don't drift again — or better, extract the pool into a small shared module both crates use, which #907 stopped short of doing.

Start by reading ruvector-core/src/storage.rs and ruvector-graph/src/storage.rs, including their directory creation, traversal checks, pool-key construction, and the Weak+Drop shape noted for the graph pool. Run the existing storage tests, then add the issue’s concurrent equivalent-path and symlink-path regression cases in both crates. Done means both spellings share one Database, see each other’s writes, and do not produce a redb lock error.