Text (character-range) comments: comment on a word / phrase / span

[adnaan]

Currently a comment can only anchor to whole lines. This adds sub-line, character-range comments: select a word, a phrase, or a span across lines — by mouse, touch, or keyboard — and comment on exactly that text.

What's new

• New kind=text comment with FromCol/ToCol (0-based, half-open, rune offsets) + the selected snippet in the anchor. CSV gains from_col/to_col (reader stays length-tolerant, 7→15 cols).
• Selectable diff: each line is now a selectable <div role=button tabindex=0> (was <button>, whose content iOS/UA make non-selectable) with a [data-line-text] container; click/Enter still select the line.
• Client directive lvt-fx:text-select (vendored bundle; source in feat(dom): lvt-fx:text-select — character-range selection over diff text client#140): floating Comment button on drag/long-press, and a keyboard block caret (visible on load; ←/→/↑/↓; Shift+arrows select; Ctrl/Alt = word).
• Persistent highlight: server-side gitdiff.MarkRanges wraps [FromCol,ToCol) in <mark class="comment-span"> (splits across chroma tokens), rendered via the zero-arg LineDisplay() map. Works with no JS, survives re-render, theme-aware.
• Side-aware composer/cards: a modified line's number exists on both the old and new rows, so rendering is gated on the selected side (fixes a double-composer).
• Drift: kind=text reuses line-anchor relocation, then relocateTextColumns re-finds the snippet in the moved line.

Why it's safe

• Load-bearing invariant guarded by gitdiff/textcontent_test.go: a rendered line's .content textContent equals the raw line rune-for-rune — the only reason browser-computed offsets match server-stored ones.
• Native copy (Ctrl+C / iOS long-press menu) is preserved — the feature augments selection, never suppresses it.

Tests

Unit: CSV round-trip + 7→15-col back-compat, MarkRanges (cross-token / unicode / fallback / merge), textcontent invariant, LineSpan/predicates, text-column drift. E2E (chromedp): mouse select→CSV, keyboard caret + select, visible-caret-moves, modified-line single-composer, mark highlight. Existing keyboard/line e2e all still green.

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