fix(store): use advisory build locks

[jleni]

Fixes #821.

What changed

• replace per-key PID marker ownership with OS advisory locks
• use one StoreLock RAII primitive for both build-key and GC locks
• keep lock files at stable paths to prevent unlink/inode split-brain races
• make waiters probe advisory-lock availability instead of polling path existence
• return immediately when a waiter finds an already committed entry
• remove PID liveness, age-based stale recovery, and the recovery meta-lock

Tests

• PID metadata is ignored for ownership
• foreign/unparseable metadata cannot steal a held OS lock
• concurrent claim has exactly one winner
• process exit releases a held key lock
• explicit guard drop unlocks even when a duplicated handle remains alive
• waiter observes commit after advisory-lock release
• waiter timeout and no-owner behavior remain bounded
• focused mutation run: 5 caught, 1 unviable, 0 missed/timeouts
• just test
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check

Tradeoffs and compatibility

Stable inode identity costs one small persistent lock file per encountered cache key. This is preferable to unlinking lock files, which can split contenders across different inodes; a bounded striped lock table remains an option if lock-file inode growth proves material.

All Kache processes sharing one local store should be upgraded together. Mixed PID-marker/advisory-lock versions cannot provide a sound transition protocol across PID namespaces.