Keep hint searches off the main actor

[austin-smith]

What Changed

Move hint searches into a session-owned actor that preserves planner caches. Add cooperative cancellation to all ten planners, reject stale results, and save hint usage when a result arrives.

Why

Hint searches could occupy the main actor for up to half a second. Searching in the background keeps gameplay responsive while protecting the current board and statistics from obsolete results.

UI Changes

Disable the Hint button and menu command while a search is pending. Other gameplay remains available and cancels the pending hint when the board changes.

Validation

• macOS and iOS simulator builds passed.
• Full macOS suite: 609 tests passed; 10 opt-in screenshot fixture generators skipped. Includes 12 new tests covering cancellation, stale results, session transitions, and cleanup.
• Autoreview returned clean; diff checks passed.

Depends on #105; targets its branch to keep this diff focused on hint responsiveness.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-15T23:16:57.245184Z	3adc472	PR opened

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