perf: list studio projects from a summary index

[Nowkoo]

Stacked on the dirty-saves PR.

The library, /music projects, the 36-project limit and deletion now use project summaries (title, track count, length, pending slot). Before, they decoded every project on the server thread. Creating a song from the library did this three times.

• Summaries are cached in a per-owner projects/<owner>/library.index file, keyed by each project file's size and modification time. save() updates it. Listing rebuilds missing, stale or damaged entries, so files edited outside the studio stay correct, and a lost cache only costs a reparse.
• Measured with 36 projects of 7,500 notes: listing drops from about 1.9 s and 40 MB to under 5 ms. The first listing after upgrading builds the index once per owner, at the old cost.
• The delete confirmation keeps a summary instead of a whole decoded project for its two-minute callback. It refuses the deletion if the title, track count, length or pending take changed. Mix-only changes no longer block a confirmed deletion.

Validation: Java 21 mvn clean verify, 293 tests, 100% instruction and branch coverage.

🤖 Generated with Claude Code

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