feat: make diff tab width configurable

[benvinegar]

Summary

• expand tabs at real terminal-cell tab stops with a new default width of 4
• add tab_width config plus -x / --tab-width CLI overrides
• propagate tab width through interactive rendering, geometry and width caches, static pager output, and OpenTUI components
• document the setting and add unit, component, static-pager, and live PTY coverage

Closes #551.

Testing

• bun run typecheck
• bun run lint
• bun run format:check
• focused unit/component suite: 214 passed
• bun test test/pty/layout.test.ts: 13 passed
• bun run test:tty-smoke: 9 passed
• bun run build:bin
• real PTY smoke test with a tabbed diff
• bun run test:integration: 55 passed; the direct-file atomic-save watch test timed out identically on a clean origin/main worktree

This PR description was generated by Pi using GPT-5.6

[greptile-apps]

Greptile Summary

Adds configurable terminal-cell tab stops throughout Hunk.

• Adds validated tab_width configuration and -x / --tab-width CLI overrides, defaulting to 4 columns.
• Propagates tab width through interactive rendering, geometry caches, collapsed rows, static pager output, and public OpenTUI components.
• Expands tabs according to terminal display width and syntax-highlight span position.
• Documents the new option and adds CLI, configuration, rendering, component, pager, and PTY coverage.

Confidence Score: 5/5

The PR appears safe to merge, with tab width consistently validated and propagated across the affected rendering paths.

Tab expansion uses terminal-cell measurements, highlighted spans preserve their cumulative display column, and tab width participates in shared row planning and geometry cache invalidation; the changed paths also include focused coverage.

Important Files Changed

Filename	Overview
src/core/tabWidth.ts	Centralizes the default, bounded runtime validation, and strict CLI parsing for tab widths.
src/core/config.ts	Reads, validates, layers, and defaults the new tab_width configuration.
src/core/cli.ts	Adds the short and long CLI overrides and carries the parsed value into common options.
src/ui/diff/codeColumns.ts	Expands source tabs at terminal-cell-aware tab stops rather than replacing each tab with a fixed number of spaces.
src/ui/diff/pierre.ts	Applies tab expansion consistently to plain and syntax-highlighted rows while preserving columns across highlight spans.
src/ui/diff/diffSectionGeometry.ts	Includes tab width in row planning and geometry cache identity, preventing stale measurements across widths.
src/ui/staticDiffPager.ts	Applies the resolved tab width to the non-interactive pager rendering path.
src/opentui/HunkDiffBody.tsx	Exposes the configurable width to embeddable rendering and defaults it consistently with the CLI application.
src/ui/App.tsx	Propagates the bootstrap tab width into line-width calculations and the interactive diff pane.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  CLI["CLI -x / --tab-width"] --> Config["Config resolution"]
  TOML["tab_width in TOML"] --> Config
  Config --> Bootstrap["App bootstrap"]
  Bootstrap --> Interactive["Interactive diff renderer"]
  Bootstrap --> Pager["Static diff pager"]
  Public["OpenTUI tabWidth prop"] --> Components["Embeddable components"]
  Interactive --> Rows["Tab-expanded diff rows"]
  Pager --> Rows
  Components --> Rows
  Rows --> Geometry["Width, wrapping, and geometry caches"]

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