MST/2 compose acceptance: finish the offline-reopen check and wire the job into CI

[Ivanbeethoven]

Context

A compose-based MST/2 acceptance stack exists in this repository:

docker-compose.mst2-e2e.yml      # layered on the repo's own docker-compose.yml
tests/mst2-e2e/Dockerfile        # bakes the mst2_mount / mst2_sync examples
tests/mst2-e2e/acceptance.sh     # the checks
tests/mst2-e2e/run.sh            # one-command runner
tests/mst2-e2e/README.md

It builds mega2 from the sibling checkout with the MST/2 surface enabled, seeds a
repository with the shapes that stress the reader, and then checks:

1. capabilities gate,
2. seeding (small file, empty file, executable, symlink, 2 MiB + 7 B file,
   129-entry directory),
3. sync + hydrate with the client's own manifest compared against a full walk,
4. mount vs git truth — every served byte against a plain git clone of the
   same commit, plus the executable bit and symlink target,
5. SYS-01 — after main advances the mounted view is unchanged and the new file
   is absent,
6. offline reopen with no server contact,
7. incremental reuse meters for the next version.

Checks 1–5 and 7 pass today. Check 6 (offline reopen inside the compose job) is
the open piece, and the job is not wired into CI.

Scope

1. Finish the offline reopen check. The store's content lives in a
   scope-level cache (DurableStore::open_with_content, shared by every view of
   the scope), so a reopen has to open the same content directory rather than a
   view-local one. examples/mst2_mount.rs now does that, but it needs to be
   exercised by the acceptance run: mount, unmount, remount with M2_STORE_DIR,
   and assert the byte listing is identical. The step must print the reopen log
   when it fails so a failure names its cause instead of showing an empty mount.
2. Make the run CI-able. The job needs /dev/fuse and a privileged
   container; where that is unavailable the runner should skip with a clear
   message instead of failing, and CI should run it wherever the device exists.
   Document which environments can run it (tests/mst2-e2e/README.md says it
   today; CI needs the same statement).
3. Keep the stack in step with upstream. The base docker-compose.yml and
   mega2-compose.yml in the sibling repo are the source of truth for the
   backend wiring; the override must not fork it beyond enabling MST/2 and
   adding the acceptance job.
4. Optional but useful: fail the run when the incremental meters regress
   (a second version of an unchanged view must not refetch everything), so the
   acceptance also guards the reuse claim.

Acceptance

• tests/mst2-e2e/run.sh exits 0 with all checks passing, including the offline
  reopen.
• The run is idempotent: a second consecutive run from a clean state passes.
• A run with a deliberately broken check fails with a message naming the check
  (no silent pass, no head-truncated output hiding a FAIL).
• CI references the job (or a documented command) and the README states the FUSE
  prerequisite.

References

• tests/mst2-e2e/*, docker-compose.mst2-e2e.yml, deploy/README.md
  (FUSE prerequisite), sibling mega2-compose.yml