feat(core): show parent function/class context in diff hunk headers

[remorses]

Parses the context text after @@ in unified diff hunk headers (e.g. @@ -10,5 +10,7 @@ function setup()) and renders it as a separator row between hunks. The @ sign appears in the gutter column alongside +/-, and the function/class name appears in the content area with the same background as context lines.

• Parse @@ context text from raw diff string and attach to each hunk
• Render hunk header rows only when context text is present (no empty @@ rows)
• Show @ in the gutter sign column, context text in content area
• Use same background colors as context lines for seamless blending
• Add hunkHeaderFg option for the @ sign color
• Add diff demo example with contextual headers
• Add tests with inline snapshots for unified and split views

Closes #587

[pkg-pr-new]

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commit: b05d420