worktree-fs refuses an absolute in-worktree destination on Windows

[SUaDtL]

The failure

worktree-fs.test.ts > symlink-safe worktree writer — refusal paths > judges an absolute destination by where it … fails on windows-latest:

UnsafeWorktreePathError: unsafe worktree path rejected
 ❯ unsafe worktree-fs.ts:26:9
 ❯ writeWorktreeFile worktree-fs.ts:75:116
 ❯ worktree-fs.test.ts:220:7

The test asserts that an absolute destination is judged by where it resolves, not by its spelling. On Windows it is refused outright.

Passes on ubuntu-latest. Passes on a developer Windows box.

How it surfaced

plugins/ca/tools had never run on Windows in CI — the tools job is ubuntu-latest only. #521's coverage-union job added a windows-latest cell; #539 fixed the containment bug that was masking everything behind it, and this is one of two failures left standing underneath.

Why it is NOT #539

#539 was validateWorktreeRoot in farm.ts comparing two spellings of one directory lexically. This is writeWorktreeFile in worktree-fs.ts, a different module with its own path judgment, and it survives the #539 fix. Filed separately so neither investigation inherits the other's lead — the same reasoning that split #504 from #495 and #515.

The likely shape is related though, and worth checking first: worktree-fs.ts does its own containment reasoning, and the runner's C:\Users\RUNNER~1\… short path plus a D:-drive workspace give it two ways to disagree with itself that a developer box does not reproduce.

Acceptance criteria

• AC-1: the cause is identified — specifically whether the refusal comes from a short-name/long-name mismatch, a drive-letter or UNC difference, or a genuine platform difference in how the destination resolves.
• AC-2: an absolute destination that resolves inside the worktree is accepted on Windows, and the test asserts its own precondition so it cannot silently stop testing that.
• AC-3: every genuinely-escaping destination is still refused on Windows, including via a junction — the refusal is a security boundary and must not be relaxed to fix a false positive. farm's worktree containment refuses a legitimate root when two path spellings disagree (Windows 8.3) #539's fix is the precedent: canonicalize, which tightened the guard rather than loosening it.
• AC-4: worktree-fs.test.ts passes on windows-latest in CI.

Not in scope

Found while closing #539. That issue's fix is complete and verified independently; this is a separate defect in a separate module.