feat: show diff hunk scope labels

Show per-side Tree-sitter scope paths inline and as sticky continuation labels.
Keep presentation rows out of source coordinates, selections, and edits.

Verified: cargo test (949 passed), cargo build, and real PTY smoke covering
split/unified, wrapping, both split axes, sticky scrolling, live rename and undo.

Size exception: 549 implementation lines added plus deleted, excluding tests
and docs. Keep syntax extraction and coordinate-safe rendering together as one
working behavior.

docs/editing.md

@@ -438,6 +438,8 @@ Review marks belong only to the open comparison and survive filtering, file navi
 
 Text panes show change excerpts with **10 unchanged lines above and below** each change, preserving original file line numbers. Overlapping or touching excerpts merge; omitted stretches show an unchanged-line count in both panes. Files with only metadata changes show their notice or “No text changes” rather than unrelated source.
 
+**Scope labels** identify named enclosing functions/classes and containers, such as `impl Review › run`. Muted, unnumbered rows appear at excerpt starts and scope changes; the current scope stays at the top when its declaration scrolls out of view. Each split side uses its own source, so renamed functions keep distinct old/new names. Unified labels use `−` for Before, `+` for After, and `±` for shared names. Narrow panes prefer the innermost name. Labels are presentation only: motions, selections, copying, editing, and line jumps retain original source coordinates. Context comes from the existing Tree-sitter trees, including supported injected code, without requiring an LSP; unsupported syntax gets no guessed label.
+
 In **Normal mode** with a text pane focused, **`[` expands above** and **`]` expands below** the excerpt containing the caret, revealing up to **15 lines** per keypress in either pane. In editable After's Insert mode these keys insert literal text. Expansion clamps at file boundaries and merges excerpts when they meet. Existing visible source stays anchored. Opening a file resets its excerpts to the configured initial context. Live edits recompute changes while retaining expanded context; the caret stays visible even when the last text change is reverted.
 
 Live typing highlights only the displayed source rows while retaining each snapshot's original syntax tree for multiline and injected-language context. Line-width caches and unchanged hunk layouts avoid repeated full-file work; edits that change line alignment or revert changes recompute the diff. Already-queued printable Insert keys share a redraw, but every key still passes through normal bindings, snippet handling, and undo history. Mode changes and other terminal events retain their order.