Add PDF box annotations with hover preview and click-to-edit

[jlongvalRobotiq]

Summary

Add rectangular PDF annotations for figures, diagrams, and other regions that cannot be anchored conveniently to a text selection. Region comments use the existing comment editor, YAML persistence, comments viewer, and Markdown export; the PDF itself is not modified.

Interaction

• b in standard PDF mode or Space+B starts box drawing. Space+b retains upstream's zen-border toggle.
• Move with hjkl/arrows, set a corner with v/Space, then confirm with Enter; alternatively left-drag a box and release.
• Type a comment and press Esc to save. Esc during drawing cancels; viewport resize/layout changes cancel unfinished geometry.
• Hover inside a saved box to preview its comment. A stationary click opens the editor on release; dragging inside the box still selects text.
• Drawing-mode shortcuts use the configurable pdf_box keybinding context. Ctrl+click/right-click retain SyncTeX behavior.

Implementation and compatibility

• Add an optional-on-disk region flag to PDF comment targets. Missing flags default to text annotations, and false is omitted when serializing. Existing storage locations and text annotation behavior are retained.
• Persist rectangles in PDF points with a top-left origin. Boxes are restricted to a single page. Text inside a box is captured as quoted context when available.
• Render outlines as four edge bars before tile clipping, preserving existing underlines, link outlines, and transparent-page rendering.
• Reuse the existing page-scoped invalidation interface rather than copying rectangle collections.
• Include each rectangle's own 1-based page and point coordinates in Markdown exports, including multi-page text annotations.
• Update README and built-in help.

Integrated current upstream main (824cc7b), including per-page enhancement/zoom geometry and pixel-precision mouse handling. Deferred selection preserves the original mouse-down position.

Verification

• cargo test --lib: 577 passed.
• cargo test --test svg_snapshots: 92 passed, 2 ignored; no golden updates.
• cargo check --all-targets
• cargo build --release
• cargo clippy --all-targets completes with warnings. Strict -D warnings is not clean on the local Rust 1.98 toolchain; for example, it flags the existing wildcard pattern in src/parsing/markdown_renderer.rs:540.

Added focused regressions for viewport-edge boxes, zoom/pan and differently enhanced pages in scroll mode, resize cancellation, click-versus-drag behavior, legacy YAML compatibility, disk-backed region persistence, text/box coexistence, per-page export locations, and outline/underline tile clipping.

Manual testing on Linux/Kitty covered drawing, saved comment preview, leaving a box/divider, and click-versus-drag editing. After integrating current main, a separate live smoke check confirmed the configurable drawing keys open the box comment editor. Cross-terminal VHS goldens were not regenerated; additional terminal coverage would be useful during review.