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>

tests/unit/coords.test.js

@@ -0,0 +1,88 @@
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+
+const {
+  physicalBoundsOf,
+  physicalToDip,
+  displayForPhysicalPoint,
+  displayForDipPoint,
+} = require('../../app/coords');
+
+const display = (id, x, y, width, height, scaleFactor = 1) => ({
+  id, scaleFactor, bounds: { x, y, width, height },
+});
+
+test('at 100% scale, physical and DIP coordinates are identical', () => {
+  const displays = [display(1, 0, 0, 1920, 1080, 1)];
+
+  assert.deepEqual(physicalToDip({ x: 640, y: 360 }, displays), { x: 640, y: 360 });
+});
+
+test('at 200% scale, physical pixels halve into DIP', () => {
+  // This is the classic marker-offset bug: a click at physical (1500, 900)
+  // on a 2x display is the DIP point (750, 450); drawing the marker at the
+  // raw values lands it far below-right of the real click.
+  const displays = [display(1, 0, 0, 1440, 900, 2)];
+
+  assert.deepEqual(physicalToDip({ x: 1500, y: 900 }, displays), { x: 750, y: 450 });
+});
+
+test('fractional scale factors convert exactly', () => {
+  const displays = [display(1, 0, 0, 1280, 800, 1.5)];
+
+  assert.deepEqual(physicalToDip({ x: 960, y: 600 }, displays), { x: 640, y: 400 });
+});
+
+test('physical bounds are DIP bounds times the scale factor', () => {
+  assert.deepEqual(physicalBoundsOf(display(1, 100, 50, 1280, 800, 2)),
+    { x: 200, y: 100, width: 2560, height: 1600 });
+});
+
+test('multi-monitor: a click on the secondary display converts in that display space', () => {
+  // Two 1920x1080 displays side by side, uniform 2x scale. Physical x=4800
+  // is the middle of the second display; its DIP x must be 1920 + 480.
+  const displays = [
+    display(1, 0, 0, 1920, 1080, 2),
+    display(2, 1920, 0, 1920, 1080, 2),
+  ];
+
+  assert.equal(displayForPhysicalPoint({ x: 4800, y: 500 }, displays).id, 2);
+  assert.deepEqual(physicalToDip({ x: 4800, y: 540 }, displays), { x: 2400, y: 270 });
+});
+
+test('multi-monitor with negative origin (display left of primary)', () => {
+  const displays = [
+    display(1, 0, 0, 1920, 1080, 1),
+    display(2, -1920, 0, 1920, 1080, 1),
+  ];
+
+  assert.deepEqual(physicalToDip({ x: -960, y: 540 }, displays), { x: -960, y: 540 });
+  assert.equal(displayForPhysicalPoint({ x: -960, y: 540 }, displays).id, 2);
+});
+
+test('a point just outside every display maps via the nearest one', () => {
+  // Clicks on the outermost pixel row can round to one pixel outside the
+  // display bounds; they must not be dropped or mapped to the wrong screen.
+  const displays = [display(1, 0, 0, 1920, 1080, 1)];
+
+  assert.deepEqual(physicalToDip({ x: 1921, y: 540 }, displays), { x: 1921, y: 540 });
+});
+
+test('no display geometry means no conversion (caller falls back to a cursor read)', () => {
+  assert.equal(physicalToDip({ x: 10, y: 10 }, []), null);
+  assert.equal(physicalToDip(null, [display(1, 0, 0, 100, 100)]), null);
+  assert.equal(physicalToDip({ x: Number.NaN, y: 10 }, [display(1, 0, 0, 100, 100)]), null);
+});
+
+test('displayForDipPoint routes a click to the containing display, else the nearest', () => {
+  const displays = [
+    display(1, 0, 0, 1920, 1080, 1),
+    display(2, 1920, 0, 1920, 1080, 1),
+  ];
+
+  assert.equal(displayForDipPoint({ x: 2000, y: 10 }, displays).id, 2);
+  assert.equal(displayForDipPoint({ x: 10, y: 10 }, displays).id, 1);
+  assert.equal(displayForDipPoint({ x: 5000, y: 10 }, displays).id, 2, 'nearest display wins for out-of-bounds points');
+});