fix: make privacy scrubbing fail closed

[abrichr]

What changed

• Validates scrubber dependencies before reading source content.
• Binds every scrub to one immutable policy snapshot and records its digest.
• Makes first-party Presidio and the documented public provider boundary fail closed under policy changes.
• Drops raw screenshot paths and pixels from text-only sanitized derivatives.
• Invalidates policy-bound analyzer, entity, and image-redactor caches.
• Removes an unavailable hosted scrubber from provider discovery.

Why

A missing dependency, mutable policy, or stale provider cache must never produce success-shaped sanitization evidence or let raw artifacts cross the boundary.

Validation

• 80 tests passed; 2 optional-dependency tests skipped.
• Ruff passed.
• Wheel and sdist built and inspected.
• Source-policy guard passed.
• Independent exact review exercised A-to-B-to-A policy changes, legacy providers, nested mutation, concurrency, stale aliases, screenshot-only flows, exception restoration, and Presidio cache invalidation.

Exact reviewed head: 413fdbedfbea7f25c60031d5fa50b4889f3347c6.

[abrichr]

Coordination hold: exact head 413fdbedfbea7f25c60031d5fa50b4889f3347c6 is independently accepted and all hosted checks pass. Do not merge yet: a fix: merge to main auto-publishes the next Privacy patch. Merge this once in the coordinated Types 0.7 / Privacy patch / Flow 1.26 release window, then verify the published GitHub and PyPI artifacts and bind Flow's optional dependency to the released fail-closed contract.