Guard the click-capture pipeline with an automated end-to-end check

Add tests/checks/test_click_capture_selftest.sh: runs the real Electron
STEPFORGE_CLICK_SELFTEST session and asserts every scenario passes — 3/3
markers at 0.00% offset, 8/8 burst clicks kept on finish, the first armed
click captured (warmup click ignored), and the debounce (4/4). Picked up
automatically by tests/run_test.sh. Skips cleanly when the host has no
capture environment so it never falsely fails CI, but fails the suite on any
real regression in click->screenshot->step behavior.

Document the guard in ARCHITECTURE.md and CHANGELOG.md.

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

docs/ARCHITECTURE.md

@@ -156,9 +156,13 @@ Reliability rules that keep "one click → one step" true under load:
   one.
 
 `STEPFORGE_CLICK_SELFTEST=1 npm start` exercises the whole pipeline in a
-real Electron session: it reports steps-per-click and marker offsets, then
-runs a fast-burst-then-finish scenario that must save every click.
-`STEPFORGE_CAPTURE_LOG=1` prints one diagnostic line per click decision.
+real Electron session across four scenarios — marker accuracy (0.00%
+offset), a fast-burst-then-finish that must save every click, the
+warm-before-arm first click, and the ~200ms debounce. It runs automatically
+as `tests/checks/test_click_capture_selftest.sh` (skipped only when the host
+has no capture environment), so a regression in click→screenshot→step
+behavior fails the suite. `STEPFORGE_CAPTURE_LOG=1` prints one diagnostic
+line per click decision.
 
 ## Security Rules
 

docs/CHANGELOG.md

@@ -36,6 +36,11 @@ Keep-a-Changelog conventions; versions follow semver.
     screenshot is late" — while every later click was fine. Now frames are
     buffering by the time the window tucks away, so the first click is
     served a pre-click frame like the rest.
+  - The whole click→screenshot→step pipeline is guarded end to end by
+    `tests/checks/test_click_capture_selftest.sh`, which runs a real Electron
+    session and asserts marker accuracy, no dropped burst clicks, the first
+    click of a session captured, and the debounce — so this behavior fails
+    the suite if it ever regresses.
 
 ### Added