feat: add shift range commenting

[osolmaz]

Summary

Keyboard users had to enter a separate selection mode before commenting on a range.
This change makes Shift-Up and Shift-Down extend a line range and opens the comment editor when Shift is released.
Saved range comments now keep a green rail beside every covered source line, so their full anchor remains visible.

What Changed

The keyboard and mouse flows now share the same range-selection model.
The terminal requests enhanced key events where supported and retains a next-key fallback for older terminals.

• Added forward and reverse Shift-arrow selection with modifier-release handling.
• Added safe keyboard-enhancement setup and restoration alongside the existing terminal guard.
• Added persistent range rails, including an active style for the selected comment.
• Kept the footer focused on save/cancel while preserving standard Emacs-style editor shortcuts.
• Updated the user controls and tmux end-to-end scenario.

Testing

Unit, rendering, architecture, and real pseudoterminal interactions pass locally.
The complete mutation, coverage, audit, documentation, and compatibility gate is clean.

• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• scripts/e2e-tmux.sh
• scripts/ci-local.sh
• cargo +1.88.0 check --workspace
• 54 unit tests and 3 architecture tests passed.
• LLVM line coverage passed at 97.38% for in-process production modules.
• 118 scoped mutants ran: 114 caught and 4 unviable, with no survivors.
• Slophammer DRY, boundaries, unsafe, and complete checks passed with no findings.

Risks

Modifier-release events depend on terminal keyboard-protocol support.
Compatible terminals open the editor immediately on Shift release; older terminals finalize the range on the first subsequent unshifted key and apply that key to the editor.

• Keyboard enhancement flags are pushed and popped through the same cleanup path used for raw mode and mouse capture.
• Mouse selection and the existing v selection mode remain available.

[osolmaz]

Final implementation report

This change adds keyboard-driven range commenting while keeping the interface focused:

• Shift+Up and Shift+Down extend the current line selection.
• Releasing Shift opens the comment editor. Terminals that do not report modifier-release events fall back cleanly when the next ordinary key arrives.
• Pressing Enter after a legacy-terminal selection opens the editor without immediately submitting an empty comment.
• Every source line covered by a saved comment displays a green range rail, so the full commented range remains visible.
• The footer no longer advertises Ctrl+A, Ctrl+E, or Ctrl+O; the underlying basic Emacs-style editing shortcuts remain available.
• The tmux end-to-end test exercises real Shift-arrow escape sequences, modifier release, comment entry, persistent range rails, and Markdown output.

Verification

• cargo test --workspace --all-targets: 54 unit tests and 3 architecture tests passed.
• cargo clippy --workspace --all-targets --all-features -- -D warnings: passed.
• scripts/e2e-tmux.sh: passed for mouse and Shift-keyboard commenting flows.
• scripts/ci-local.sh: passed.
• Line coverage: 97.38%.
• Mutation testing: 118 mutants evaluated; 114 caught and 4 unviable, with no survivors.
• Dependency audit: no known vulnerabilities.
• Slophammer and SimpleDoc checks: clean.
• Rust 1.88 compatibility check: passed.
• Codex review: no P0 or P1 findings. The sole P2 finding was fixed and covered by a regression test.
• GitHub Actions: both verify and rust 1.88 passed.