Revalidate output roots and write targets during Plan.Apply

[dremnik]

Problem

projection.BuildPlan validates the output root and all expected targets, but Plan.Apply revalidates only stale removals. Creates and updates proceed through replaceFile without confirming that the output root and target still have the type and ownership observed during planning.

Between planning and applying, another process can:

• create an unmanaged file at a target that was previously missing, which is then overwritten; or
• replace the validated output directory with a symlink, allowing writes outside the originally canonicalized root.

This breaks the filesystem-safety guarantees under a time-of-check/time-of-use race.

Relevant code

• internal/projection/writer.go: BuildPlan, Plan.Apply, and replaceFile
• tests/projection/writer_test.go: removal revalidation exists, but write/root revalidation does not

Expected behavior

Immediately before each write, verify that the output root still resolves to the planned root and that the target still satisfies the planned create/update precondition. Abort without overwriting an unmanaged or redirected path.

Acceptance criteria

• A planned create refuses an unmanaged file introduced before Apply.
• A planned update refuses a target whose ownership or type changes before Apply.
• Replacing the output root with a symlink before Apply cannot write outside the planned root.
• Existing stale-removal revalidation continues to pass.