Preserve changed lines that resemble unified diff headers

[Maya-Kid]

Why

DiffView and patchStats discarded every line beginning with +++ or ---, including genuine hunk content. For example, changing --count; to ++count; produces such prefixes in a real Git patch, so changes disappeared and their statistics were undercounted.

What changes

Share a small line-filtering helper between rendering and statistics. Track hunk line counts to distinguish changed content from file headers, reset at Git file boundaries, and keep support for compact tool-output hunks. Document the input boundary: complete unified hunks can distinguish file headers by their counts; multi-file compact or truncated output needs diff --git separators. Keeping changed text takes precedence when shortened output omits that information. The public API and styling are unchanged.

How it was checked

• Fresh npm ci, npm run docs, npm run typecheck, and npm run build pass on Node 26.3.0.
• Chrome 155: complete /dev/examples reports 389/389 mounted and 0 failed in both themes, without page errors.
• Generated real patches with git diff --no-index and compared counts with Git's --numstat: prefix-like code/Markdown lines, --- literal / +++ literal hunk text, empty added/deleted lines, missing final newlines, and multiple files all retain their changed lines and correct counts.
• Checked abbreviated hunks and the existing compact tool-output example, plus CRLF, omitted and zero counts, header context, and compact/truncated multi-file output retaining diff --git boundaries.
• Checked StepGroup's aggregate statistics and expanded diff. Visually inspected the rendered content, line colors, and open tool step in both themes.

Before review

• Every commit is signed off (git commit -s)
• npm run typecheck and npm run build pass
• /dev/examples reports 0 failed
• What the change touches was looked at in both themes, open states included
• A component of Solid Glass's own has its example in src/examples/ and its page in content/solid/