Add floating capture pill

[r3dbars]

What

Implements WS1.4's single detected-meeting prompt surface as a small AppKit capture pill:

• New CapturePillPanel / CapturePillController with app icon, meeting name, Record, and Not now.
• sharingType = .none, .canJoinAllSpaces, and .fullScreenAuxiliary so the pill can appear over full-screen apps while staying out of screen capture.
• 30-second auto-dismiss routes through the same Not now/backoff path; no countdown auto-record was added.
• Escape dismisses; Return records; VoiceOver labels are set on the pill and controls.
• Detected meeting prompts now route through the capture pill instead of the meeting recording overlay prompt.

Checks

• python3 scripts/dev/check-build-source-lists.py ✅
• git diff --check ✅
• bash run-tests.sh --filter OverlayScreenSharePrivacyTests ✅ (9 passed)
• bash build-deps.sh --force ✅
• bash build.sh --no-open ✅ (signed app + launch smoke + performance budget)

Manual proof still needed

• Trigger a calendar/runtime/browser detected-meeting prompt on a real Mac and verify the pill layout, button actions, Escape/Return behavior, and 30-second Not now auto-dismiss.
• Start a real screen share or ScreenCaptureKit capture while the pill is visible and verify the pill is not captured.
• Verify appearance over a full-screen app/space.
• VoiceOver pass: focus reads the meeting capture prompt and both actions clearly.

In-flight PR check

Open PR search found related meeting/menu-bar work (#1355, #1351), but no open PR that implements WS1.4's floating capture pill.

Lane closeout

COORD_DONE: YELLOW | this PR | capture pill implemented + detector wiring + privacy/keyboard contract coverage | checks above green | manual UI/screen-share/VoiceOver proof needed | lanes used: Codex=implementation/build/tests/PR; Claude=skipped, narrow AppKit implementation after local inspection; Local=attempted scan via ~/.codex/maestro/runs/20260703T001548Z-ws14-pill-scan-local but returned no repo findings; Windows=skipped, local macOS AppKit build/proof needed | smallest next action: run the real Mac prompt + screen-share visibility pass.