test(tern): pin the one-data-plane-apply-per-deployment invariants

[aparajon]

Why this matters

The northstar's core invariant — one data-plane apply per deployment, with sibling operations attaching into it — is enforced across four PRs (dispatch keying, attach echo, fail-closed persistence, read model). This capstone pins the whole chain end to end so a regression in any layer fails a test that names the invariant, not a distant symptom.

What it does

• A real in-process gRPC dispatch test drives both shard operations of one deployment: both requests carry the same deployment-keyed idempotency key, both operation rows record the deployment's one remote apply id, and the parent apply's external_id stays untouched.
• A status-endpoint test proves the deployment-filtered fold surfaces the shared data-plane apply id on the folded row while keeping per-operation remote ids out of it.
• The task-store mock now scopes GetByApplyOperationID to the requested operation the way the real store does, so multi-operation dispatch tests exercise genuine per-operation task sets.
• Control-plane drive semantics for the shared apply — a completed sibling never terminalizes the parent, ordered finalizer gating, concurrent same-deployment shard drives — are already pinned by the operator multi-operation integration matrix; this PR adds the identity-correlation coverage on top rather than duplicating it.

How it moves us toward the northstar

One data-plane apply per deployment; operations dispatch into it.

0  per-shard reconcile                          ✅ shipped
pre vschema-only deployment-scoped shape        ✅ shipped
pre op-lease drives settle projection-safely    ✅ shipped
1  dispatch granularity
   ├─ data-plane sibling attach                 ✅ merged
   └─ deployment-keyed idempotency + echo       ✅ merged
2  correlation persistence                      🔍 in review
3  op-scoped data-plane drive (shared apply)    ✅ pinned here + existing matrix
4  read model exposes the deployment apply id   🔍 in review
5  CLI renders it as APPLY ID in deployment mode 🔍 in review
6  test matrix                                  ⬅ this PR

Opened by Claude (Fable 5).