feat: complete native Capture producer contracts

[abrichr]

Outcome

This change completes the Capture producer contract for native recording across Windows, macOS, and native X11 Linux.

• keeps one fixed encoded viewport while a recorded window moves, resizes, or crosses monitors
• binds each pointer and keyboard action to one exact ScreenEvent, WindowEvent, and geometry generation
• detects process replacement, recycled window handles, mixed frame geometry, stale action geometry, and display-topology changes
• adds native X11 EWMH window capture and fails closed on Wayland until a stable portal can provide the same identity contract
• adds macOS Accessibility and modern GI AT-SPI structural observers beside the existing Windows UIA observer
• exposes strict public v2 window-event models and preserves exact bindings through processed actions
• removes static Experimental, Beta, and Development Status metadata; the signed production admission ledger owns release selection
• keeps the repository Chrome extension as an excluded, repository-only prototype; Flow Playwright remains the supported browser recorder

Verification

• 476 passed, 10 skipped with the exact hosted CI test command on macOS
• 474 passed, 2 skipped, 10 deselected for the combined non-slow suite
• 76 passed, 2 skipped for focused geometry and processing tests
• 20 passed, 1 skipped for focused structural tests
• Ruff passed for every changed Python file
• wheel and sdist built and passed the release archive verifier
• both archives contain the X11, macOS AX, and Linux AT-SPI producer modules
• both archives exclude the browser bridge and Chrome extension
• wheel metadata exposes the Linux extra and contains no Development Status classifier

Release boundary

This PR does not publish a release or change an active admission. Interactive Linux, macOS, and Windows hardware qualification and exact Flow consumer admission must complete before the signed ledger selects these bytes.