Advances #3021 Phase 1 — test-only, no version bump. Extends the single-store Conjoined + Quick + UseTenantPartitionedEvents fixture with the remaining aggregate-handler scenarios that don't need a new harness.

New coverage

tenant_partitioned_aggregate_matrix_phase1b (string identity):

• Exclusive-write concurrency — [AggregateHandler(ConcurrencyStyle.Exclusive)] → FetchForExclusiveWriting; appends to the routed tenant, stays isolated.
• Empty result → no write — handler yields nothing; stream + aggregate unchanged.
• MartenOps tenant overloads — Store/Insert/Delete(doc, tenantId) land in the targeted tenant's document partition vs default, invoked with no ambient tenant.
• DeliveryOptions{TenantId} routing → correct partition.
• AlwaysEnforceConsistency — concurrent write on the same tenant stream → ConcurrencyException.

tenant_partitioned_natural_key_aggregate (Guid stream identity, string natural key — reuses NkOrderAggregate/NkOrderHandler):

• Same natural-key value in two tenants resolves to the routed tenant's own stream (FetchForWriting<T, TNaturalKey> is tenant-scoped, not cross-tenant).
• Multi-event return appends to the routed tenant.
• [WriteAggregate] IEventStream<T> handler completes in the routed tenant and stays isolated.

Findings

• No new bugs in the covered scenarios — all routing/isolation behaves correctly under partitioning.
• One genuine gap surfaced and filed separately as IEventStream<T> compound-handler append is silently dropped without AutoApplyTransactions (inconsistent with [AggregateHandler]/IMartenOp after GH-3025) #3032: a compound handler that loads an IEventStream<T> via FetchForWriting and appends gets no SaveChanges without AutoApplyTransactions (silently dropped), inconsistent with [AggregateHandler]/IMartenOp (the latter self-persists since MartenOps.StartStream side-effect is silently dropped under Events.UseTenantPartitionedEvents #3025). Not partitioning-specific (confirmed against a plain store; Bug_225 only passes with AutoApplyTransactions). That shape is intentionally omitted here and annotated in the matrix; the [WriteAggregate] IEventStream<T> aggregate-workflow path (exercised by the natural-key CompleteNkOrder test) is unaffected and persists fine.

Verification

• Targeted: phase1b 6/6, natural-key 3/3.
• Full MartenTests suite: 486/486 (one transient tracking flake on a first run did not reproduce on a clean re-run).

Still deferred on #3021 Phase 1

Message-member-bound tenant routing (no first-class core attribute found — needs a design call), the HTTP [Aggregate] matrix (needs an Alba host), IEventStream<T> loader shape (pending #3032), and remaining return shape IEventStream<T> non-aggregate. Phases 2–3 unchanged.

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