fix(storage): release recovery writer locks on error exits (#1283)

[DecisionNerd]

A cleanup resource-limit error dropped the parent writer-lock file without explicitly unlocking it. A duplicated or fork-inherited descriptor could therefore keep the kernel lock alive and make the next writer receive GF_WRITER_BUSY.

Return a private writer-lock guard from recovery-lock acquisition. Its error-path destructor explicitly unlocks; successful paths retain fallible explicit release and close the file before project-root deletion. All four retention/recovery callers preserve checkpoint-before-writer release order and allocation accounting.

Two deterministic regressions keep a duplicate of the production writer descriptor alive across max_entries and max_bytes_scanned errors. Both failed on the prior implementation with GF_WRITER_BUSY. They require independent writer acquisition and a repeated public cleanup preview while the duplicate remains open. Existing genuine-contention coverage remains enabled. This proves the ownership defect; fork inheritance as the trigger of the earlier CI occurrence remains an inference.

Validation: cargo test --locked --release -p graphforge-storage project_retention::tests -- --test-threads=4 passed all 15 tests. The same optimized storage test executable passed all 29 recovery tests with four test threads. The pre-fix duplicated_writer_lock filter failed both tests as expected. Workspace Clippy, formatting and make pre-push-fast pass; independent review found no actionable findings. No public API, durable-format, RSS-policy, or scale-workload change.

Closes #1283. Prerequisite for completing #1278 via PR #1281.

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