fix: rebalance Pierre word highlights in split view

[benvinegar]

Summary

• keep Pierre word-level highlights visible in split view by deriving inline emphasis backgrounds from the diff row colors when theme defaults are too subtle
• stop the fallback blend at the first visible shade, with finer blend steps so the highlight stays noticeable without looking too bright
• add model- and render-level tests for inline word-diff emphasis and record the user-visible fix in CHANGELOG.md

Testing

• bun run typecheck
• bun test

This PR description was generated by Pi using OpenAI o4-mini

[greptile-apps]

Greptile Summary

This PR fixes invisible word-level highlights in Pierre split-view by introducing a wordDiffHighlightBg helper that checks whether the theme's existing addedContentBg/removedContentBg has enough contrast against the row background, and if not, blends incrementally toward the sign color until a minimum Manhattan distance of 28 is reached. It also consolidates the blendHex utility (previously local to renderRows.tsx) into a new shared src/ui/lib/color.ts module alongside the new hexColorDistance helper.

Confidence Score: 5/5

• This PR is safe to merge — the logic is correct, the type narrowing is sound, and both model-level and render-level tests are in place.
• No P0 or P1 findings. The strengthening loop correctly returns the first blend that meets the threshold, the StackLineCell["kind"] subset is compatible with the SplitLineCell["kind"]-typed cache, and the module-level cache is bounded to the small fixed set of themes. All remaining observations are P2 style notes.
• No files require special attention.

Important Files Changed

Filename	Overview
src/ui/lib/color.ts	New shared color utility module exporting blendHex (moved from renderRows.tsx with input validation added) and the new hexColorDistance (Manhattan RGB distance); both implementations are correct.
src/ui/diff/pierre.ts	Adds wordDiffHighlightBg + strengthenWordDiffBg to derive a visible inline word-diff background; replaces the raw theme-color lookup in both makeSplitCell and makeStackCell. Logic and typing are correct — StackLineCell["kind"] is a strict subset of SplitLineCell["kind"] so the shared cache Record is safe.
src/ui/diff/renderRows.tsx	Removes the local hexToRgb/blendHex helpers now that they live in color.ts; all call sites updated to import from the shared module. Clean refactor with no behaviour change.
src/ui/diff/pierre.test.ts	Updates the model-layer split-view test to assert that word-diff spans carry a background (any, not the exact theme constant) and that non-word spans don't; correctly reflects the new strengthening behaviour.
src/ui/components/ui-components.test.tsx	Adds a render-level integration test that measures the actual pixel background distance between word-diff spans and their surrounding spans, asserting ≥ 28 Manhattan units; polling loop correctly waits for async highlighting. Minor: spanIndex <= 0 silently skips the case where the marker lands at span index 0, but the test input ensures "41"/"42" never do.
CHANGELOG.md	Appends a user-visible fix entry to the [Unreleased] ### Fixed section as prescribed by the project conventions.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["wordDiffHighlightBg(kind, theme)"] --> B{Cache hit\nfor theme.id?}
    B -- Yes --> C["Return cached[kind]"]
    B -- No --> D["Check hexColorDistance(\naddedContentBg, addedBg\n) ≥ 28?"]
    D -- Yes --> E["addition = addedContentBg"]
    D -- No --> F["strengthenWordDiffBg(\naddedBg, addedSignColor\n)"]
    F --> G["Loop: blend ratio 0.005→0.2\nstep 0.005"]
    G --> H{distance from\nlineBg ≥ 28?}
    H -- Yes --> I["Return candidate\n(early exit)"]
    H -- No, more steps --> G
    H -- No, loop done --> J["Return max-blend\ncandidate (0.2)"]
    E --> K["Build cache record\naddition / deletion / context / empty"]
    F --> K
    J --> K
    I --> K
    K --> L["wordDiffBackgroundCache\n.set(theme.id, cached)"]
    L --> C

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[benvinegar]

@copilot resolve the merge conflicts in this pull request