feat(input): add Visual Line mode (V)

[roramirez]

Summary

Adds a vim-like linewise Visual mode. V (from Normal or Visual) enters Visual LINE, where the selection always spans whole lines regardless of the cursor column: j/k extend it line by line, y copies every selected line, v returns to characterwise selection, Esc exits. The status bar badge reads V-LINE.

The mouse path is unified too: triple-click (select_line_at) now leaves the user in Visual LINE, so a mouse-picked line can keep being extended with j/k. Its immediate copy yields identical text to before.

Changes

• InputMode::Visual gains a linewise: bool field, carried unchanged through every reconstruction of the variant (motions, scroll, boundary scroll, swap anchor).
• New Action::VisualLineAnchor + AppState::set_visual_line_anchor(), routed from both dispatch_visual_action and dispatch_action. A dedicated action is required because do_set_mode discards keybinding-supplied fields on a Normal→Visual transition, which would silently drop the flag.
• "V" bound in both the Normal-mode handler and visual_char_action; "v" now explicitly clears linewise, making it the way back to charwise.
• Only two computations are linewise-aware:
  • Highlight — selection_range in src/renderer/text.rs normalizes to (0, start_row, cols-1, cur_row), so nothing downstream in the render pipeline changes.
  • Copied text — do_visual_copy passes sc = 0, ec = cols-1; grid.selected_text's existing row_col_range already makes interior rows full-width, giving exact vim linewise semantics (trailing-space trim, \n join). grid.rs is untouched.
• mode_style renders V-LINE when linewise, reusing the same theme.palette[5] color as VISUAL.
• select_line_at builds its mode with linewise: true.
• Docs: CHANGELOG.md, README.md, doc/SPEC.md, and the Visual Mode section of doc/LLMs.md.

How to test

cargo fmt --check
cargo clippy --locked -- -D warnings
cargo test

1245 tests pass. New coverage:

• src/input/keybindings_test.rs — V yields VisualLineAnchor from Normal and Visual; all motions (h/j/k/l/0/$/g/G, arrows, Home/End) preserve linewise; v returns to charwise. Modifier combos per CLAUDE.md: Ctrl+V and Ctrl+Shift+V do not produce VisualLineAnchor, Shift+V in Insert does not, and Alt+V is asserted to fall through like every other Visual char key (pre-existing behavior for the whole Visual keymap, documented in the test).
• src/app_state_test.rs — VisualLineAnchor yields anchored: true, linewise: true from both Normal and Visual; VisualAnchor clears the flag; a multi-row linewise Copy returns full rows ("hello\nworld") rather than the column-clipped span; boundary scroll, viewport scroll, and swap-anchor all preserve linewise; and V+y over one row produces byte-identical text to the triple-click path.
• src/renderer/text_test.rs — a linewise selection highlights full rows without panicking; mode_style reports V-LINE with the same color as VISUAL.

Manually: run mmterm, Ctrl+. twice into Visual, press V, extend with j/k, y to copy, then paste to confirm whole lines. Triple-click a line and press j to confirm it extends.

Note: cargo clippy --all-targets reports pre-existing lints (useless_vec, dropping_copy_types) in test files untouched by this branch; the CLAUDE.md-mandated cargo clippy --locked -- -D warnings is clean.