perf: only write unsaved studio projects on disconnect and shutdown

[Nowkoo]

Stacked on the note-memory PR.

Cached projects already match their files, except a final take that is still saving or whose write failed. Disconnects and shutdown now retry only those.

• Before, every disconnect rewrote the player's project, and shutdown rewrote every cached project on the single writer thread. With many players that could reach the 5-second shutdown timeout, with a genuinely unsaved take queued behind redundant writes.
• A disconnect now evicts a current project from memory; the next visit reloads it.
• "Take saved" is announced only for finished takes. It is no longer sent to everyone online at shutdown.

Validation: Java 21 mvn clean verify, 289 tests, 100% instruction and branch coverage.

🤖 Generated with Claude Code

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[Nowkoo]

Consolidated into #28.