Own text resources and clarify region selection state

[devmobasa]

Summary

This refactor replaces ambient text resources with explicit ownership and consolidates region-selection state.

• retain separate UI layout and shape measurement owners across rendering, input, toolbar, session, asynchronous, and export paths
• make help-overlay interaction geometry owner-scoped
• cache help content using the runtime inputs that affect it
• centralize region-selection lifecycle, ownership, phase, and geometry
• split picker, review-action, cut-history, and preview responsibilities
• remove temporary thread-local resource bridges and internal compatibility adapters

No user-facing behavior change is intended.

Resource ownership

Runtime paths now reuse persistent UiTextEngine and TextMeasurer owners. Standalone public convenience APIs remain available where practical and create one call-local owner per operation.

This keeps mutable text caches isolated while avoiding repeated resource construction in internal loops.

Public API change

The ambient help hit-map functions were removed:

• clear_help_overlay_hit_map
• help_overlay_region_at

Standalone interactive callers should use render_help_overlay_result and query the returned HelpRenderResult::hit_map with HelpHitMap::region_at.

Region selection

The backend now owns the canonical selector lifecycle and geometry. Input state retains only a synchronized projection used for gating and painting.

The refactor preserves capture, OCR, measurement, review, cut editing, cancellation, stale-result rejection, and device ownership behavior.

Validation

Validated on Rust 1.98.1:

• RUST_TEST_THREADS=1 ./tools/lint-and-test.sh
• all-feature core tests: 4,534 passed, 1 ignored
• no-default-feature core tests: 4,376 passed, 1 ignored
• configurator tests: 481/480 passed
• CLI, daemon fixture, UI integration, and doc tests passed
• clippy passed with -D warnings in both feature configurations
• Rust and Nix GitHub checks passed