Clarify file section layout helper names

[benvinegar]

Summary

• rename the section layout helper module to make its file-specific scope explicit
• rename the exported layout helpers and local DiffPane variables to match that scope
• keep the sticky-header and viewport-anchor behavior unchanged while making the geometry code easier to follow

Testing

• bun run typecheck
• bun test test/app-interactions.test.tsx test/ui-components.test.tsx test/ui-lib.test.ts test/section-heights.test.ts

This PR description was generated by Pi using OpenAI o3

[greptile-apps]

Greptile Summary

This PR is a pure renaming refactor: src/ui/lib/sectionLayout.ts is renamed to fileSectionLayout.ts, all three exported functions and the exported interface gain a File/FileSection prefix to make their file-specific scope explicit, and every call site and local variable in DiffPane.tsx is updated consistently. No logic, algorithm, or behavioral changes are made.

• SectionLayoutMetric → FileSectionLayoutMetric
• buildSectionLayoutMetrics → buildFileSectionLayoutMetrics
• findHeaderOwningSection → findHeaderOwningFileSection
• resolveFileHeaderTop → resolveFileSectionHeaderTop
• All four corresponding local variables in DiffPane.tsx renamed to match
• No other files in the codebase imported the old module, so the rename is complete and self-contained

Confidence Score: 5/5

Safe to merge — purely a naming refactor with no functional changes.

All renamed symbols are updated consistently in every consumer. No other file imported the old module, no barrel re-exports exist, and no test files reference the old names. The function implementations and all call-site logic are byte-for-byte identical to the pre-rename code.

No files require special attention.

Important Files Changed

Filename	Overview
src/ui/lib/fileSectionLayout.ts	Renamed from sectionLayout.ts; exports renamed to FileSectionLayoutMetric, buildFileSectionLayoutMetrics, findHeaderOwningFileSection, and resolveFileSectionHeaderTop — no logic changes.
src/ui/components/panes/DiffPane.tsx	Import path and all symbol usages updated to match the renamed module; local variables renamed from sectionLayoutMetrics/baseSectionLayoutMetrics/selectedSectionLayout to their file-scoped equivalents — no behavioral changes.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[DiffPane.tsx] -->|imports| B[fileSectionLayout.ts\n renamed from sectionLayout.ts]
    B --> C[buildFileSectionLayoutMetrics\n was buildSectionLayoutMetrics]
    B --> D[findHeaderOwningFileSection\n was findHeaderOwningSection]
    B --> E[resolveFileSectionHeaderTop\n was resolveFileHeaderTop]
    B --> F[FileSectionLayoutMetric\n was SectionLayoutMetric]
    A --> G[baseFileSectionLayoutMetrics\n was baseSectionLayoutMetrics]
    A --> H[fileSectionLayoutMetrics\n was sectionLayoutMetrics]
    A --> I[selectedFileSectionLayout\n was selectedSectionLayout]

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