Add time-travel debugging (record button + timeline slider)

[NullVoxPopuli-ai-agent]

Time-travel debugging

This PR adds a Time Travel tab to the inspector: a record button and a timeline slider that scrubs the inspected app's state back and forth.

(mp4 / webm versions of the demo)

How it works

Ember has no single state atom, but long-lived app state overwhelmingly lives in tracked/settable properties on container-managed singletons. The new ember-debug/time-travel-debug.js module:

• Record: while recording, hooks _backburner.on('end') (the same "runloop settled" signal render-debug uses) and captures a snapshot of every serializable property of all service:/controller: singletons in the container cache. Consecutive identical states are deduplicated; the buffer is capped at 200 snapshots.
• Travel: writes a snapshot's values back through set() / tracked setters (only properties that actually differ), which triggers Glimmer revalidation — the app re-renders itself at that point in time. A guard prevents the restore's own runloops from being recorded.
• Captured properties: own enumerable data properties, plus prototype-chain accessors that have both a getter and a setter (the shape @tracked produces). Getter-only accessors are derived state and are skipped. Only plainly serializable values (primitives, dates, plain arrays/objects) are recorded — class instances are rejected so restoring can never corrupt identity-bearing objects.

The UI side is a standard tab (route/controller/template + nav entry) talking over the existing port protocol (timeTravel:* messages). The record button and clear button live in the toolbar; the slider sends timeTravel:travel on input, so scrubbing restores state live as you drag. The timeline shows the snapshot index, capture time, and the URL it was captured at.

Known limitations (also explained in the tab's empty state)

• Component-local state, Ember Data records, and non-serializable objects are not restored.
• The router URL is captured for display but not restored (transitioning would re-run model hooks).

Testing

• New tests/ember_debug/time-travel-debug-test.js: records tracked controller state, travels back/forward and asserts values are restored, verifies dedup of unchanged runloops and clear.
• Full Ember Debug suite passes (84 tests); eslint/template-lint/build clean.

Notes from building the demo

The demo runs the bookmarklet build injected into classic-test-app (Ember 7). Two pre-existing issues bite late injection into webpack-built Ember 7 apps and had to be worked around in the demo harness (not addressed in this PR):

1. entrypoints/utils/has-ember.js never resolves: there is no ember barrel module and no window.Ember in Ember 7, and the Ember window event fired long before injection.
2. object-inspector.js's GlimmerValidator.tagFor/trackedData patching throws (Cannot set property tagFor of #<Object> which has only a getter) because the webpack module namespace is getter-only — same problem the Vite guard (globalThis.emberInspectorApps) already dodges.

Happy to file these separately.

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