feat(diff): context collapse threshold + word wrap

[hewigovens]

Summary

Three commits carved out of #46. The word-wrap commit needs manual testing — there is no automated end-to-end coverage for the GPUI/AppKit text layout.

1. perf(jj-diff): keep short context runs uncollapsed

• Don't collapse runs of unchanged context <= COLLAPSED_CONTEXT_THRESHOLD (2) lines — render them inline instead of as an "N unmodified lines" stub.
• Rename separator placeholder from "N hidden lines" → "N unmodified lines" for consistency.
• Multi-line signature for compute_file_diff_full; new threshold tests.
• Review fixes: merged duplicate early-return blocks; dropped unreachable if hidden == 0 guard.

2. feat(diff): word wrap for unified and side-by-side diff views

GPUI shell:

• New shell/gpui/src/diff/wrap.rs — wrap_diff_lines, wrap_sbs_rows, wrap_cols_from_bounds, selection_cols_in_fragment, visual_index_for_line.
• Updated diff_view/unified_body.rs, diff_view/sbs_body.rs, diff_view/mouse.rs, selection.rs, annotate_view.rs.
• log/find.rs translates find-match line indices to visual rows. SBS path chains sbs_line_to_row (pairing-aware) → visual_index_for_sbs_row so find-next in SBS centers on the correct wrapped row.

SwiftUI shell (JayJayDiffUI package):

• New NativeDiffView+WrappedGutter.swift.
• Updated NativeDiffView.swift, DiffGutterTextView.swift, DiffTextContainerView.swift, DiffTextSupport.swift, NativeDiffView+Gutter.swift, SideBySideDiffRendering.swift to support wrapped rendering with gutter alignment.
• DiffTextContainerView now exposes wrapsText: Bool (default true). SBS (SideBySideDiffView) sets both panes' containers to false — per-side wrapping would desync the panes and gutters until we ship "wrap to tallest side + continuation rows". Unified view still wraps.
• DiffLayoutManagerTests.swift updated.

3. fix(core): rename backout → revert_change and land on @

Crown-jewel fix carved out of #46:

• repo.backout() mis-passed --insert-after rev, leaving the revert as a child of the change being reverted instead of on top of @. Rename to revert_change and pass --onto @ to match jj revert semantics.
• uniffi + Swift call sites renamed: JayJayRepo.revert_change, ChangeActions.revertChange, callers in DAGView, RepoContentView+CommandPalette, and the RepoViewModel impl.
• real_jj_repo test rewritten: now asserts the working-copy description stays at child change, the revert change appears in log, and its parent is @.
• Docs (CONTRIBUTING.md, README.md, docs/index.html) updated to the revert terminology.

Test plan

• cargo test -p jj-diff (perf threshold cases)
• cargo test -p jayjay-core (incl. revert_change_uses_jj_revert_and_creates_reverse_change)
• cargo clippy -p jj-diff -p jayjay-gpui --all-targets -- -D warnings
• just test-gpui and just test-app
• just build (full Swift app + regenerated uniffi bindings)
• Manual: open a wide diff in unified and SBS modes (GPUI + SwiftUI), resize the pane, verify wrap + gutter alignment + selection + find-jump.
• Manual: trigger Revert Change from DAG context menu and the command palette; verify the new revert change lands as a child of @.

[gemini-code-assist]

Code Review

This pull request updates the diff engine to use histogram line matching and introduces 'tiny-gap auto-expansion' to prevent collapsing small sections of unmodified lines. Feedback focused on simplifying the context collapsing logic by removing redundant checks and improving safety with saturating_sub for line count calculations.