Add inline script activation-time discovery (PEP 723 PR 8/16)

[StellaHuang95]

Draft fork-review PR. Do not send upstream yet.

Summary

• discover valid cached inline-script environments after activation
• expose discovered entries through getEnvironments('all')
• reconcile add/remove changes and emit manager environment events
• keep activation non-blocking with deferred, bounded bootstrap retries
• keep explicit refresh() a settled, single-pass operation

Safety and lifecycle

• validates sidecars, base interpreters, venv launchers, direct-child ownership, and cache locks
• preserves transient, unavailable, and busy entries rather than deleting them
• removes stale published entries when their launcher or metadata becomes definitively invalid
• coalesces overlapping scans and cancels bootstrap retries on explicit refresh or disposal
• fixes Windows discovery to verify both the cached launcher and its base interpreter

Scope

This PR does not add automatic routing, project registration, user-facing setup UI, cache deletion, TTL eviction, or lifecycle telemetry.

Validation

• TypeScript test compilation
• ESLint
• focused unit suite: 176 passing, 3 expected platform skips
• three manual review passes; no significant findings remain

[StellaHuang95]

Closing this PR in favor of a clean-history replacement with the same current reviewed file changes.

[StellaHuang95]

Superseded by clean-history replacement #9.