Harden replay synchronization and add focus-at verb

[jserv]

Stabilize the motif-fileview-done smoke by anchoring the in-process snapshot on activity quiescence via wait-converge instead of a fixed delay.

Add a focus-at replay verb that resolves the X subwindow under a target-local point and calls XSetInputFocus on it via the main thread, exposing Motif's XmToggleButton focus-highlight rect that synthetic ButtonPress alone does not trigger.

Cover the SDL_USEREVENT round-trip surface in src/snapshot.c with shared helpers (prepareSnapshotRoundTrip, signalSnapshotResult, waitSnapshotResult), an SDL_atomic event-type, and a generation counter so a request that times out cannot have its stale event satisfy a fresh waiter. All three round-trips (snapshot, resize, focus-at) now carry a SnapshotEnvelope with generation + payload.

Mirror timelineTapXEvent inside popPutBackEvent so synthesized crossing chains and send-event payloads appear in timeline JSONL. Track alreadyTapped on PutBackEvent so events that already went through convertEvent's terminal tap (drainSdlEventsToPutBack) are not double-counted.

Tighten the wait-converge parser to reject trailing garbage; on timeout/divergence fall through to the runner's expected state-snapshot line instead of breaking out so the runner errors fast instead of polling 31s for a marker that would never arrive.

Add DEBUG-gated XLoadQueryFont / XCreateFontSet trace lines so the empirical font-parity investigation under the differential harness is reproducible without re-editing source.

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Summary by cubic

Stabilizes replay sync by waiting for UI quiescence and adds a new focus-at verb to set X input focus on the correct subwindow. Hardens SDL round-trips with generation-tagged envelopes, improves event timeline coverage, tightens wait-converge parsing and failure handling, and fixes wheel scroll by pairing press/release to match X11.

• New Features

  • Added focus-at x y to focus the subwindow under a target-local point (runs on the main thread). For xdotool, falls back to windowfocus and now requires a prior wait-window or errors.
  • Enhanced wait-converge: stricter validation, rejects trailing garbage, accepts failure-marker=path in any position. The runner writes a state-snapshot after it and writes/reads the failure marker to fail fast without breaking the script.

• Refactors

  • Unified snapshot/resize/focus-at round-trips with a generation-tagged SnapshotEnvelope, atomic SDL event type, and shared helpers; stale events no longer satisfy new waits.
  • Timeline: put-back events are now tapped once; alreadyTapped prevents double counts and synthesized/send-event paths are tapped on delivery. Added a regression test for deferred Expose.
  • Input: SDL_MOUSEWHEEL now emits ButtonPress + matching ButtonRelease so Motif scroll actions fire; tests drain both.
  • Debug: added logs for XLoadQueryFont and XCreateFontSet. Minor runner error-message cleanups and stricter focus-at arg checks.

^{Written for commit ba52ece. Summary will update on new commits.}