Harden Playground cache retention safety

[chubes4]

Summary

• replace PID-based ownership with token-owned renewable leases carrying host, boot, PID, and process-start identity
• bind heartbeat renewal and removal to open file/directory generations so expired owners cannot overwrite or remove replacement leases
• reject and safely unlink non-directory .refs sidecars without following symlinks outside the cache
• protect symlinked and multiply-linked archives with lstat, separate logical/estimated removal metrics, and verify reclaimed bytes from filesystem free-space delta
• clean orphan lock/reference sidecars while preserving active foreign leases
• run bounded maintenance before archive resolution/download and retain post-failure/teardown cleanup diagnostics
• wire unit, process, concurrency, attack, and lifecycle integration coverage into exact-head CI

Regression coverage

• expiry/reacquire/old-heartbeat and old-release generation race
• separate-process shared leases, crash expiry, PID reuse, and foreign host/namespace ownership
• .refs symlink acquisition, archive cleanup, and orphan-sidecar attacks
• hard links, archive symlinks, and open-unlinked allocation accounting
• concurrent lock/reference acquisition versus maintenance
• stale sidecars from the previously merged implementation
• cache cleanup ordering before the next archive request
• startup/teardown evidence, environment policy, invalid policy, bounds, and idempotence

Validation

• npm run build
• npm run test:playground-custom-archive-cache
• npm run test:production-boundary-enforcement
• npm run test:agent-task-contracts
• exact workflow integration commands: runtime sources, readonly PHPUnit cache, native Playground E2E/interruption, trusted apply, redaction, and runtime tool policy
• existing Playground release cache, readonly mount, bootstrap INI, and preview lease regressions
• git diff --check

The full npm run check still reaches only the unchanged command-registry smoke failure tracked in #1745. This is a follow-up to merged #1929 and contains only commits absent from current main.