docs(spec): refine ClearCam design from review

Zero runtime permissions: minSdk 29 removes the WRITE_EXTERNAL_STORAGE
branch. Clarify filters are ML Kit UI-provided (not app-customizable).
Make multi-page OCR async with progress and defined page-join order.

Constraint: app requests no runtime permissions (no camera, no storage).
Confidence: high
Scope-risk: local
Reversibility: trivial
Directive: user reviews spec, then writing-plans.
Tested: design self-review + advisor review.
Not-tested: no code yet.
Related: ClearCam greenfield

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