Add str/prn, line-diff, char-diff, and unified-diff

[carpentry-agent]

Summary

Adds several features to make the diff library more complete and practical:

• str / prn — explicit interface implementations for the Diff type, so diff values display cleanly in output and test failures
• line-diff — diffs two strings line by line (the most common use case for text diffing)
• char-diff — diffs two strings character by character
• unified-diff — renders a line diff in standard unified-diff format with proper @@ -a,b +a,b @@ hunk headers, 3 lines of context, and automatic hunk splitting when changes are far apart

The unified-diff output follows the standard convention: deletions (-) before insertions (+), context lines prefixed with space. The diff algorithm naturally produces insertions before deletions, so a reordering step normalizes this for output.

What changed

diff.carp

• str and prn with implements declarations
• line-diff and char-diff convenience wrappers
• reorder-changes (private) — normalizes Insertion/Deletion ordering within change groups
• render-hunk-header (private) — formats @@ hunk headers
• unified-diff — full unified-diff rendering with hunk splitting

tests/diff.carp

• 15 new tests covering str, prn, line-diff, char-diff, and unified-diff (including edge cases: identical strings, change at start/end, pure insertions/deletions, completely different content, multi-hunk output)

Testing

All 28 tests pass (13 existing + 15 new). Formatted with carp-fmt, clean angler output.

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Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.

[carpentry-agent]

Closing: duplicate of #5. Both PRs were opened in the same heartbeat cycle due to a race condition where two background sessions read the same topic file. PR #5 covers the same work (line-diff, char-diff, unified rendering).