Fix diff line counts and GitHub-style stats

[cursor]

Summary

• Add shared diff line-stat parsing that counts additions/deletions from hunk bodies, including new and removed files.
• Render GitHub-style green/red per-file stats in diff headers while preserving hunk pills for summaries and multi-hunk navigation.
• Update affected diff Storybook snapshots.

Resolves ENG-518

Test Plan

• bun -F native test:unit -- diff-line-stats.test.tsx
• bun -F native lint
• bun -F native build
• bun -F native test:storybook -- src/components/widget/summaries/hunk-pill.stories.tsx src/components/widget/summaries/full-file-diff-editor.stories.tsx src/components/widget/summaries/diff-section.stories.tsx

Note: full bun -F native test:storybook still has unrelated pre-existing snapshot flakes in animated rebuild/file-view stories after installing Playwright Chromium.

Docs

• Docs updated (companion PR in darkmatter/nixmac-web: #___)
• No docs update needed

  

[github-actions]

	Fails
🚫	New UI components were added without a Storybook story. Add a sibling *.stories.tsx file: apps/native/src/components/widget/summaries/diff-line-stats.tsx

	Warnings
⚠️	PR is marked WIP / draft — do not merge until ready for review.
⚠️	Please assign this PR to someone (usually yourself).

📋 PR Overview

Lines changed	198 (+161 / -37)
Files	2 added, 5 modified, 0 deleted
Draft / WIP	yes
Has Test Plan	yes
New UI components	yes (1)
New Storybook stories	no
New Rust modules	no
New TS source files	yes (1)
New tests	yes (1)
package.json touched	no
Cargo.toml touched	no
Infra / CI touched	no

🔬 Coverage

Report	Lines	Statements	Functions	Branches
apps/native/coverage/coverage-summary.json	18.1%	18.1%	31.3%	54.9%

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