prisma-risk / tsoracle / 26430611421

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feat(driver-openraft): joiner gate and migration-on-next-write tests (#465)

This closes the migration-on-next-write half of the zero-downtime format
migration and adds the (currently latent) capability pre-check for
learner admission.

**Migration-on-next-write (`state_machine.rs`).** Earlier phases threaded
the process-shared active-write-version cell into the snapshot encoder,
so `build_snapshot` already stamps the cell at build time rather than a
hard-coded constant. This commit pins that with four tests so a
regression to a literal trips compilation/assertion immediately:

- `build_snapshot_stamps_active_write_version` — the persisted envelope's
  leading version byte (and the inner snapshot payload's leading byte)
  equal the accessor's value, asserted against `sm.active_write_version()`
  not a literal so the test stays correct after a future activation
  moves the cell forward.
- `old_version_snapshot_is_read_and_reemitted_at_active_version` — a
  snapshot persisted by one SM is recovered by a fresh SM and the next
  `build_snapshot` emits at the cell's current value. Honest about the
  seam: while `MIN_READABLE_VERSION == MAX_READABLE_VERSION ==
  BASELINE_WRITE_VERSION`, this exercises end-to-end at the only
  version that exists; a genuine cross-version rewrite becomes testable
  when a real higher version parser ships.
- `snapshot_codec_accepts_full_readable_range` — the framed codec
  accepts every version in `[MIN, MAX]` and rejects `MAX + 1` loudly.
- `active_write_version_survives_reopen_and_drives_writes` — the
  durable value is stable across reopen and is exactly what the
  subsequent build stamps.

**Joiner gate (`lifecycle/membership.rs`).** A pure capability
pre-check for learner admission, plus a thin enforcement wrapper.
LATENT on this branch: the admin path on the merged tree calls
`Raft::add_learner` directly, not the toolkit's wrapper. A future
admit caller that routes through `add_learner_gated` will turn this
into a live r... (continued)

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