Keep Presentation cancellation available during startup

[niharnm]

Presentation disabled every session button while preparing or starting, leaving its direct view without a cancellation control. Cancel Presentation now remains available beside progress, with Escape invoking the same action. It calls the existing stop path, which waits for pending work and restores applied changes.

Cancelled window receipts are retained before cancellation is handled, so recovery remains possible and failures remain visible. Cancellation is unavailable during recovery. Existing active-session End and Restore and confirmed Keep Current Setup behavior is unchanged.

Validation:

• Source review found and corrected cancellation being interpreted as a partial window failure.
• Added regression coverage for preparation visibility, startup cancellation, completed and cancelled window receipts, protected recovery, and recovery failure ownership.
• git diff --check passed.
• All 26 isolated Presentation controller tests passed without launching Semper.
• Cancellation regressions use cancellation-aware arrival/event signals, a one-minute test limit, and guaranteed held-task release and drainage. A delayed Scene regression covers observer cancellation before Workspace entry.
• Debug xcodebuild build-for-testing passed with two build jobs and a separate DerivedData directory.
• GitHub CI run 34374508846 passed on the revised test-synchronization head 629ef7c.
• Signed native validation of the button, Escape, and accessibility behavior remains required before acceptance.

The change is isolated from the current accepted candidate at fb9a3d5. File Shelf and Window Layout are separate work.

[vercel]

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Project	Deployment	Actions	Updated
semper	Ready	Preview	Sep 9, 2026 4:05pm UTC
semper-vmf1	Ready	Preview	Sep 9, 2026 4:05pm UTC

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