RT-300: parse a file-to-symlink typechange's two diff blocks

[m4ttheweric]

RT-300. When a file is replaced by a symlink (or back), git emits two diff --git blocks for that one path: a delete, then an add. The vendored GHD DiffParser threw "Invalid hunk header format" on the second header, so glitter could neither show nor stage the file.

What changed

git-core

• new parseFileDiff (diff-hunks.ts) parses each block alone and joins the hunks, shifting unifiedDiffStart/End so every line keeps a unique selection index
• stagingDiff and commitDiff both use it and set typechange: true on such a diff

glitter

• buildDiffModel shows a typechange diff read-only (no stage bar, no line or hunk toggles)
• it stages whole from its file-row checkbox, since half a file and half a symlink is not a state git can stage

Guards (from the Opus review standing in for rate-limited CodeRabbit)

• stageSelection and discardSelection refuse a typechange (a leftover Partial reached git apply and wrote a corrupted regular file)
• the commit-time index rebuild drops such a Partial to None and refuses the commit with a notice naming the file, so a retry commits the rest
• blocks split at \n only, so a lone \r before diff --git in a content line never splits a block
• commitDiff and the range diff pass -M, so a rename never reads as two blocks

Verification

• sandbox tests: working-tree stagingDiff (hunks, unique indices, sources, stageFileFully gives 100644 -> 120000 T), a committed typechange through commitDiff, and the model's read-only flag; all three failed before the fix
• live in glitter on a scratch repo: the diff shows -one -two -three then +other.txt, the file-row toggle works, committing gives :100644 120000 ... T f.txt, and History shows the committed typechange
• git-core plus mission suites 567/567

🤖 Generated with Claude Code

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