submodule: associations follow checkout, inherit on create, merge on fold

[ed-irl]

Make tracked submodule branch associations move with branch operations transactionally, with HEAD rollback on conflict.

• branch checkout (and friends): after switching branches, switch each tracked submodule to the branch recorded for the target. The checkout handler snapshots HEAD first and rolls parent + already-switched submodules back to their original HEADs if the apply fails. Wiring is optional — a nil Submodule applier disables auto-checkout.
• branch create: a fresh child inherits its parent's recorded submodule associations as a baseline (RecordWithInheritance), then overlays the current worktree state so an explicitly-moved submodule wins.
• branch fold: merge the folded child's submodule records into the surviving base (MergeAssociationsForFold). Disjoint and identical records carry over silently; conflicting per-submodule records resolve via the new repeatable '--module-branch=PATH=BRANCH' flag, an interactive prompt, or fail with a FoldConflictError listing all conflicts.
• branch onto: apply the target's submodule associations after BranchOnto runs (it bypasses the checkout handler), snapshotting parent HEAD for rollback on failure.

Adds checkout.GitWorktree.SnapshotHead/RestoreHead, a checkout SubmoduleApplier interface, and unit + test-script coverage for inheritance, checkout apply, and fold merge.

Part of #1246

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat(submodule): track and manage submodule branch associations #1178
    • feat(submodule): add git-layer primitives and transactional/recursive handlers #1179
      • submodule: associations follow checkout, inherit on create, merge on fold #1180 ◀
        • feat(worktree): add worktree commands, per-worktree trunks, and worktree filtering #1181
          • feat(worktree): add worktree commands, per-worktree trunks, and worktree-aware filtering #1182
            • feat(worktree): Add worktree commands, per-worktree trunks, and worktree-aware filtering #1183

_{Change managed by git-spice.}