Multi-client time travel debugger for lamdera live

[CharlonTank]

What

A full-stack, multi-client time travel debugger built into lamdera live, opened from the devbar ("Time Travel").

• Unified event timeline — every client broadcasts its state mutations (frontend msgs, backend msgs, ToBackend/ToFrontend) with the resulting wire-encoded model over the live websocket. Any tab can inspect every client's FrontendModel and the BackendModel at any recorded instant, with per-client colour badges and ToBackend session/client provenance.
• Master-tab scrubbing — selecting a past frame (or dragging the slider) makes every connected client display its own state at that instant, with a banner and interactions neutralised. Works across profiles, incognito windows and devices since the bus rides the existing websocket (the server already re-broadcasts unrecognised messages, so no server changes were needed for transport). Broadcasts are debounced so fast slider drags only land the final position, and clients that didn't exist yet show their init state.
• Global restore — "Restore all here" forks the whole system (all frontends + backend) back to a chosen point; timeline truncated, app keeps working from there.
• Detached popup viewer — "⧉ Pop out" opens a real popup window that takes over from the inline panel (which hides, and comes back when the popup closes). It receives the full history on open via its opener, and acts as a remote control. It deliberately has no websocket, so it never counts as an app client (no onConnect/onDisconnect noise).

Recording is O(1) per event (frames hold references to the immutable models; Debug.toString only runs lazily for rendered rows/sections), history is capped at 1000 frames.

Implementation notes

• New extra/LocalDev/runtime-src/Lamdera/TimeTravel.elm module + integration in Live.elm (record points at the four state mutations, bus handling, viewer mode).
• extra/live.js: websocket bus relay (base64 for bytes), popup open/close lifecycle, opener→viewer postMessage relay, history dump.
• Embedded-asset staleness fix (extra-source-files + addDependentFile + dev-build.sh content stamp): editing live.js or the runtime harness now actually rebuilds the embedded copies — plain cabal build used to say "Up to date" and silently ship stale assets.
• test/scenario-time-travel/: demo app (backend-synced counter + purely local counter + onConnect sync) exercising all five event kinds.

How to test

./dev-build.sh
cd test/scenario-time-travel
$(cabal list-bin lamdera) live

Open a couple of tabs plus an incognito window, click both counters around, then hover the devbar → "Time Travel": scrub the slider (all windows follow), pop the panel out, restore to a past frame.

Verified with Playwright end-to-end suites covering single-tab (26 checks) and multi-client incl. incognito context, popup lifecycle, fast-scrub stress and global restore (27 checks).