fix(core): batch untracked file diffs instead of spawning git per file

[Hona]

A working-tree diff ran 2 git processes per untracked file, one after another (statUntracked + patchUntracked). In a worktree with 257 untracked eval files, /api/vcs/diff ran longer than the app's 60 s request timeout. The app retried endlessly, page loads waited behind the stuck requests (blank tab), and the server eventually stalled.

This is a performance-only change: output is byte-identical to v2.

flowchart LR
  L[files v2 would diff one by one] --> E{regular file, not huge?}
  E -- no --> F["per-file v2 command (unchanged)"]
  E -- yes --> A["GIT_INDEX_FILE=$TMP (empty) git add -N --pathspec-from-file"]
  A --> D["git diff --numstat / --patch against $TMP"]
  D --> M{"chunk header == diff --git a/f b/f?"}
  M -- yes --> OK[use batched result]
  M -- no --> F

Loading

• status() and files() keep the v2 flow. Before the loop, untrackedBatch prefetches results for exactly the files v2 would pass to statUntracked / patchUntracked. Each lookup falls back to the original per-file command.
• The throwaway index starts empty and holds only those files. The user's index is never read or written. Tracked files, git rm --cached paths and nested repos keep their v2 handling.
• git diff against an intent-to-add entry applies the same attributes as --no-index (text=auto, clean filters, -diff, binary, working-tree-encoding), so the patch text matches byte for byte.
• Anything the batch can't reproduce exactly falls back: directories, symlinks, files over 5 MB, quoted paths, non-default diff prefixes, files git add rejects.
• git add runs with --literal-pathspecs, --force, --sparse and -c core.splitIndex=false.

Benchmark (257 untracked files, Windows)

	v2	this PR
git processes started	783	17
diff("working")	28.4–34.0 s	0.34–0.36 s
status()	14.1–14.3 s	0.24–0.26 s

Parity

400 randomized repos, comparing status() and diff() (working, branch, committed, context: 1) between v2's adapter and this one:

	result
identical output	400 / 400
.git and index unchanged	400 / 400
git processes started (total)	26,877 → 17,984

The repos covered .gitattributes, git rm --cached, feature branches, nested repos, diff.mnemonicPrefix, repos without commits, staged-then-deleted files, intent-to-add-then-deleted files, core.splitIndex, symlinks, 6 MB and 11 MB files, sparse checkout, and subdirectory scopes.

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