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/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