fix(session): keep process snapshots monotonic

[guicybercode]

Context

The post-merge macOS quality job for #40 failed twice in PTY lifecycle cleanup, but in different tests. Local exact repetitions stayed green while parallel suite repetition reproduced StopTimedOut in three cleanup tests.

Root cause

Process snapshots are scanned outside the global cache lock. An older scan could finish after a newer root-proving scan and overwrite the cache. A Session ProcessTree could then consume that older snapshot, permanently prune its root identity, skip SIGKILL delivery, and time out.

Changes

• assign every process scan a strictly monotonic AtomicU64 sequence
• publish a cache entry only when its sequence is newer
• reject cached snapshots older than the scan that proved a ProcessTree root
• add deterministic unit coverage for out-of-order publication and pre-root cache rejection
• use the production-like reap deadline in the force-kill timing test while preserving the assertion that kill completes before the supervisor interval

Validation

• cargo test -p cli-master-session: 85/85
• cargo clippy -p cli-master-session --all-targets -- -D warnings: pass
• cargo fmt --all --check: pass
• git diff --check: pass
• full PTY suite stress: 20 sequential runs, 620/620 tests
• concurrent PTY suite stress: two parallel loops of 10 runs, 620/620 tests

Follow-up to #40.

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