Rearchitect click capture: strict click-time frames, off-main-process recorder, exact marker coordinates

Implements the architecture change from ai_prompts/prompt3.md:

- New app/click-frames.js: shared timestamped frame ring + strict
  click-to-frame pairing (never a frame whose grab started after the
  click); legacy slack behavior kept behind capture.strictClickFrames=false.
- New stream capture backend (app/stream-backend.js + hidden worker
  window): per-display desktop media streams sampled into ring buffers
  and PNG-encoded entirely off the main process, so click delivery is
  never starved by capture work. Auto-degrades to the legacy in-process
  frame loop when streams cannot start or the worker stops answering.
- Clicks are paired with their frame at event time (eager pairing in
  enqueueClickCapture); only the storing is serialized, so slow encodes
  cannot skew later clicks in a fast burst.
- Linux watcher: restored event-time root coordinates from
  xinput test-xi2 and merge raw/regular twin events structurally.
- Replaced the 40ms time debounce with source-aware duplicate
  suppression: fast legitimate clicks are never dropped.
- New app/coords.js: physical-to-DIP conversion with multi-monitor and
  scale-factor handling; Windows keeps screenToDipPoint.
- STEPFORGE_CLICK_SELFTEST end-to-end hook: 3/3 clicks become steps via
  the stream backend with 0.00% marker offset on this host.
- Tests rewritten/added: strict selection, coords, stream backend,
  Linux coordinate parsing, twin merge, burst clicking (126 passing).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/settings.js

@@ -19,6 +19,15 @@ const DEFAULT_SETTINGS = {
     captureOutsideClicks: true,
     confirmSimpleCapture: false,
     autoIntervalSec: 5, // session fallback when click capture is unavailable
+    // Strict click timing: a step never uses a frame whose grab started
+    // after the click. Turn off only if captures are too slow to keep a
+    // pre-click frame buffered (re-enables the legacy slack heuristics).
+    strictClickFrames: true,
+    // Off-main-process frame recorder (hidden worker window sampling a
+    // desktop media stream). Falls back to the in-process loop when false
+    // or when streams cannot start on this desktop.
+    streamCapture: true,
+    frameSampleMs: 100, // stream backend sampling cadence
   },
   editor: {
     focusedViewDefaultForNewSteps: false,