docs: add implementation checklist

IMPLEMENTATION_PLAN.md

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+# Implementation Plan
+
+Constraints: local commits only; no push; no browser testing; one commit per feature.
+
+## Controller and targeting
+
+- [ ] Keep the controller visible for its own video in element and ancestor fullscreen.
+- [ ] Target popup actions at the frame represented by the displayed speed; keep “all videos” intentional.
+- [ ] Ignore shortcuts originating from editable controls, including shadow-DOM inputs.
+
+## Accessibility and usability
+
+- [ ] Give in-player controls accessible names, keyboard behavior, and visible focus.
+- [ ] Make control-bar customization operable by keyboard as well as drag and drop.
+- [ ] Label generated shortcut and site-rule form controls.
+- [ ] Improve popup status announcements, focus indicators, and icon-search semantics.
+
+## Settings safety and validation
+
+- [ ] Confirm before Restore Defaults removes preferences and remembered data.
+- [ ] Report partial imports accurately when custom icons cannot be restored.
+- [ ] Reject malformed slash-prefixed regular expressions before saving site rules.
+
+## Extension lifecycle and copy
+
+- [ ] Initialize and synchronize the disabled toolbar icon from background state.
+- [ ] Correct shortcut, subtitle-nudge, live-update, and obsolete troubleshooting copy.
+- [ ] Run automated tests in the release workflow before packaging.
+
+## Verification
+
+- [ ] Run focused automated checks after each non-trivial change.
+- [ ] Run the complete non-browser test suite and review the final local commit series.
+- [ ] Leave cross-site fullscreen visual verification for reporter/user validation.