Patch() parser has no fixture for a hunkless rename or a mode-only change, so the design's "headers and no hunks" behaviour is unasserted #186

What's missing

The patch-verbs design commits to two parser behaviours that no test pins down:

Plan:

Renames and mode changes. Both produce a file entry with headers and no hunks. Nothing special is needed beyond not assuming every file has them.

Fixtures cover: a modification with two hunks, a file with no trailing newline, a rename with no hunks, a binary file, a combined conflict hunk, and content with CRLF line endings.

What exists today

• GitIntegration.Test/Fixtures/ has patch-rename-modified.txt (a rename with hunks) and patch-quoted-rename.txt, but no pure (100% similarity) rename fixture and no old mode / new mode fixture.
• GitPatchRoundTripTests.ReportsAStagedRenameAsDeleteAndAddUnlessRequestedAsync checks the kind and OriginalPath of a pure rename, but never asserts Hunks is empty, nor what Header keeps.
• GitIntegration/Parsing/GitPatchParser.cs recognises rename from/rename to, new file mode and deleted file mode, but has no explicit handling or test for old mode / new mode header lines (a chmod +x with no content change).

Why it matters

A mode-only change and a pure rename are the two ordinary cases where a file entry has no @@ lines. If a later parser change starts assuming each file has a hunk (or swallows the next file's diff --git line as header), a staging UI built on Patch() would lose or mis-attribute the file, and nothing in the suite would fail.

Acceptance criteria

• Fixtures captured from a real git: patch-rename-pure.txt (git mv with no edit, -M) and patch-mode-change.txt (chmod +x only), each followed by at least one further ordinary file in the same output.
• Pure rename parses as Renamed, OriginalPath set, Hunks empty, Header retains the similarity index / rename from / rename to lines.
• Mode-only change parses as a single Modified entry, Hunks empty, old mode / new mode kept in Header.
• Neither entry is flagged IsBinary or conflicted, and the following file still parses with its hunks.

Dependencies

None. Related to #126 (rename headers in PatchFor), but not blocked by it.