tray: Discard New Build, cached instead of wasted

[m4ttheweric]

Adds a way to clear the dev tray's "new build ready" state without restarting into the build. The discarded build goes into the build cache first, so rebuilding the same tree state later is a copy, not a build.

What changed

Menu (AppDelegate.swift)

• Adds Discard New Build under the Restart item

Discard (DevBuild.swift, DevBuildWatcher.swift)

• Adds DevBuild.discardScript: files the staged bundle under its cache key, then clears staged/
• An unkeyed build, or a cache that can't be written, is just deleted
• Splits the handoff's filing and tidying (sweep, drop gone trees, keep four) so both scripts share them

Verification

Two new checks in DevBuildChecks.swift; swift run mattstack-checks 546/546 green. Not yet exercised in a running dev app.

🤖 Generated with Claude Code

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 86f2936 and c394c98.

📒 Files selected for processing (5)

• rt-tray/Sources-core/Services/DevBuild.swift
• rt-tray/Sources/AccessibilityIDs.swift
• rt-tray/Sources/AppDelegate.swift
• rt-tray/Sources/DevBuildWatcher.swift
• rt-tray/Tests/MattstackCoreChecks/DevBuildChecks.swift

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