Allow custom rendering of unified diff lines

[HarshPopat23]

Summary

Add an optional line_writer callback to sourcemeta::core::Diff::FormatOptions allowing consumers to customize the rendering of individual unified-diff lines.

Background

This enables consumers (such as the JSON Schema CLI's fmt --check command) to style unified diff lines (e.g. coloring deleted lines red, inserted lines green, hunk headers cyan, etc.) while keeping the Core diff engine decoupled from terminal detection, ANSI escape sequences, and color policy.

Details

• Introduces sourcemeta::core::Diff::FormatOptions::LineType enum class:
  • HeaderOriginal (prefix = "--- ", content = <original_label>)
  • HeaderModified (prefix = "+++ ", content = <modified_label>)
  • Hunk (prefix = "@@ ", content = "-<orig> +<mod> @@")
  • Context (prefix = " ", content = <line>)
  • Delete (prefix = "-", content = <line>)
  • Insert (prefix = "+", content = <line>)
  • NoNewline (prefix = "\\ ", content = "No newline at end of file")
• Introduces LineWriter callback type: std::function<void(std::ostream &stream, LineType type, std::string_view prefix, std::string_view content)>.
• Hunk headers are formatted as a single logical line (prefix = "@@ ", content = "-<orig> +<mod> @@").
• Core retains ownership of the trailing \n.
• The default path (when no callback is set) remains direct and allocation-free.
• Appending line_writer to Diff::FormatOptions preserves aggregate initialization and designated initializer source compatibility for existing callers.
• Adds comprehensive unit test coverage in test/diff/diff_line_myers_test.cc.

[cubic-dev-ai]

No issues found across 3 files

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[augmentcode]

🤖 Augment PR Summary

Summary: This PR adds per-line customization hooks for unified-diff rendering.

Changes:

• Adds FormatOptions::LineType to identify headers, hunks, context, inserts, deletes, and missing-newline markers.
• Adds an optional LineWriter callback to Diff::FormatOptions.
• Routes unified-diff headers and content lines through the callback when configured.
• Keeps ownership of each trailing newline in the core formatter.
• Preserves the direct, allocation-free default rendering path when no callback is supplied.
• Builds hunk metadata as callback content only when custom rendering is enabled.
• Documents callback lifetime and line-body responsibilities in the public API.
• Adds coverage for semantic types, labels, multiple hunks, missing newlines, stream formatting, and embedded NULs.

Technical Notes: The callback is appended to the options aggregate to retain existing source initialization patterns while allowing consumers to apply their own styling policy.

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[HarshPopat23]

PTAL: @jviotti ,
Guide me what to add befor pr in jsonshema,
sourcemeta/jsonschema#878

[jviotti]

Looks great to me! I'll merge and let me upgrade it myself on the CLI in a little bit, as I'm managing other upgrades

[jviotti]

See sourcemeta/jsonschema#881

[jviotti]

@HarshPopat23 Upgrade done! Feel free to rebase your other PR