feat(input): double-click selects the word under the cursor

[roramirez]

What it does

A left double-click selects the word under the pointer, leaves it highlighted in anchored Visual mode (so you can see exactly what was picked), and copies it to the clipboard. Esc or a click returns to Insert.

A double-click is registered when the second press happens within ~400 ms and less than ~4 px of the previous one. Word boundaries reuse the existing is_word logic (alphanumerics and _). Double-clicking a blank cell selects nothing.

Changes

• src/input/motion.rs: new word_bounds helper (+ 4 tests).
• src/input/mouse_ops.rs: new select_word_at method — sets anchored Visual selection over the word and copies it; empty/blank cells select nothing.
• src/app_event.rs: is_double_click + wiring in handle_selection_click.
• src/app_state.rs: new field last_click: Option<(Instant, f64, f64)>.
• CHANGELOG.md.

How to test manually

1. In this worktree: cargo run.
2. Produce some text: echo hola-mundo foo_bar baz.
3. Double-click on foo_bar → the whole word stays highlighted (_ included) and is copied to the clipboard.
4. Paste (Ctrl+Shift+V or middle-click) at the prompt → foo_bar appears.
5. Press Esc (or click elsewhere) → the highlight clears and you are back in Insert.
6. Double-click on blank space → nothing is selected/highlighted and it does not crash.
7. Regression:
   • Click-and-drag still selects a free range and copies it on release.
   • A single click still moves focus / opens a URL under the cursor as before.
   • Two slow clicks (>400 ms) or far apart (>4 px) behave as two single clicks, not a double-click.

Tests

cargo test — full suite green. word_bounds covered by 4 tests (word_bounds_spans_whole_word_from_middle, _spans_first_word_from_edge, _on_space_selects_single_cell, _treats_underscore_as_word). The timing detection (is_double_click) and the on-screen highlight are validated manually (they depend on real winit/rendering).