Fix: Windows: os.replace onto in-use cached binary raises PermissionError

[pullapprove5]

The finding is real: on Windows, os.replace(tmp_path, binary_path) in both plain-tailwind/plain/tailwind/core.py:173 and plain-code/plain/code/oxc.py:195 raises PermissionError (sharing violation) when a second checkout tries to install/download the same version binary while a first checkout is actively executing it from the shared machine-wide ~/.cache/plain cache. POSIX is unaffected because os.replace there succeeds even onto an open/executing file. I confirmed the exact behavior by exercising the real Tailwind.download() and OxcTool.download() code paths end-to-end (mocked network I/O, real os.replace monkeypatched to raise the actual Windows sharing-violation errno/message) with the destination binary pre-populated to simulate "another process already installed and is running it": on the pre-fix code this raised PermissionError; after the fix it succeeded and left the pre-existing binary in place. I fixed both call sites identically: wrap os.replace in try/except PermissionError, and only re-raise if the destination binary_path doesn't already exist (i.e. genuinely still missing, not just contended) — matching the deferred finding's suggested approach. Ran ./scripts/fix on both plain-tailwind and plain-code (after installing uv via pip since it wasn't preinstalled in this sandbox) — both passed clean with zero diff (ruff, oxlint/oxfmt, prettier all no-op). Could not run ./scripts/test: it requires a Postgres server via scripts/start-postgres, and docker isn't available in this sandbox; additionally neither plain-tailwind nor plain-code has a tests/ directory or appears in scripts/test's ALL_PACKAGES list, so the repo's own suite never covered this code before or after the change. One commit was made covering both fixes.

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Opened by a PullApprove implementation run (implement-finding v4) for:

• PA-6 — Windows: os.replace onto in-use cached binary raises PermissionError

Merging this is what closes them as fixed.