#1784 fix: stabilize private AX settle snapshots

Summary

Project private-AX snapshots to the visible viewport, retaining excluded content only as scroll hints.

Stabilize settle observations by pinning private AX after fallback and comparing the visible semantic projection with bounded geometry tolerance. Geometry-less semantic nodes remain visible to agents but cannot become interaction targets.

This touches 8 files across the iOS private-AX fallback and the shared settle runtime; scope stayed within snapshot capture and settle behavior. No public CLI or skill behavior changed.

Validation

• pnpm check:affected --run passed: 1,942 affected tests and 100% changed-line coverage.
• Built the iOS runner with unit tests enabled and ran the two focused private-AX XCTest regressions twice: 2/2 passed with zero failures.
• Proved the settle backend-pinning regression red by temporarily removing the production forwarding, then restored the fix and verified green.
• Deleted the disposable verification simulators after the run.