fix(cli): keep shared dotrain meta until the last uri drops it

[thedavidmeister]

Closes #172

The defect

dotrain_cache maps uri -> hash and the hash is content-determined (keccak of the cbor encoded DotrainV1 item), so two uris holding identical text necessarily share one cache entry. Both removal paths dropped that entry without checking whether another uri still pointed at it:

• delete_dotrain(uri, keep_meta = false) — documented as deleting a record, one uri.
• set_dotrain(text, uri, keep_old = false) — the different-hash branch removing old_hash.

The result was a dangling uri: get_dotrain_hash still returned the hash, get_dotrain_meta returned None.

The fix

One private remove_unreferenced_meta guard, called from both sites once dotrain_cache no longer maps the departing uri to the hash. The entry is removed only when no remaining uri maps to it, so the last uri off a hash still takes its meta with it and the single-uri behaviour both flags already had is unchanged.

QA

• Discriminating tests: meta::tests::test_store_dotrain_shared_meta_survives_delete, meta::tests::test_store_set_dotrain_keeps_shared_old_meta — each fails on base (verified by reverting remove_unreferenced_meta to the base body self.cache.remove(hash); and re-running: both report assertion failed: store.get_dotrain_meta("b.rain").is_some(), 14 passed / 2 failed; restoring the guard returns 16 passed / 0 failed).
• Mutations applied: two, both on crates/cli/src/meta/mod.rs remove_unreferenced_meta, each run over the whole meta::tests::test_store set (16 cases).
  1. Guard dropped, back to the base unconditional self.cache.remove(hash); → 14 passed / 2 failed, killed by test_store_dotrain_shared_meta_survives_delete and test_store_set_dotrain_keeps_shared_old_meta (assertion failed: store.get_dotrain_meta("b.rain").is_some() in each). The pre-existing test_store_delete_dotrain_keep_meta and test_store_set_dotrain_branches survive it — that is the gap this PR closes.
  2. Body made a no-op, never remove → 12 passed / 4 failed, killed by the pre-existing test_store_delete_dotrain_keep_meta and test_store_set_dotrain_branches AND by the second half of each new test (assertion failed: store.get_meta(&hash).is_none() / store.get_meta(&shared).is_none()), so the fix cannot degenerate into never freeing the cache.
• Oracle: the doc contract quoted in the issue, not the implementation — Store "just maps the hash of the content to the given uri" (many-to-one, so identical text under two uris is one cache entry), delete_dotrain "deletes a dotrain record given a uri" (scoped to the one record), and get_dotrain_meta "get the corresponding meta bytes of the given dotrain uri if it exists". Expected values are the hashes set_dotrain returns for each uri, compared for equality rather than recomputed from the code under test.
• Category check: issue asks for the dangling-uri state on two units — delete_dotrain's keep_meta = false branch and set_dotrain's keep_old = false different-hash branch; both are covered, one test each. The issue's triage framing left the contract open between reference-checking and documenting the flags as unsafe; this takes the reference-checking option, which is the one that keeps the three doc statements above true, and updates both doc comments to state the resulting contract.

Tests

Two new cases in crates/cli/src/meta/mod.rs; the existing test_store_delete_dotrain_keep_meta and test_store_set_dotrain_branches are unchanged and still pass.

cargo fmt --all --check, cargo clippy -p rain-metadata --lib --all-features -- -D warnings and the 16 meta::tests::test_store cases are green locally. The full suite was not run locally by instruction (shared machine).

🤖 Generated with Claude Code

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• crates/cli/src/meta/mod.rs

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