feat: event upcaster registration extension point (ADR-010 follow-up, ADR-090)

[ivanball]

Ships the consumer-side mechanism ADR-010 named as follow-up work: the event upcaster registration extension point.

What's new

• IEventUpcaster / IEventUpcaster<TSource, TTarget> (Application): a pure payload mapping from a retired integration-event contract to its successor; the non-generic members come from default interface implementations so authors write one method.
• EventUpcasterRegistry: indexes by source type, follows chains (V1 -> V2 -> V3) by declared target type, preserves MessageId/DateOccurred across every hop (inbox dedup keys survive by construction), and throws at construction, naming the offenders, on duplicate sources, self-maps, or cycles. An internal hosted service resolves it at host start so misconfiguration fails startup, not the first message.
• services.AddEventUpcaster<TSource, TTarget, TUpcaster>(): TryAddEnumerable idiom (same shape as AddScheduledJob), accumulates across modules.
• Both delivery paths consult the registry: DomainEventDispatcher upcasts before selecting IIntegrationEventHandler<> (covers monolith mode and pre-upgrade outbox rows; domain-handler dispatch untouched), and broker hosts drain a retired contract via RegisterUpcastedIntegrationEventConsumer<TOld>() backed by the new dedicated UpcastingIntegrationEventConsumer<TOld> (existing consumer hot path and shipped ctor untouched).
• Two new fitness functions on EventConventionTestsBase, inherited by every consumer tree with no edit: unique upcaster source types, and target SchemaVersion strictly higher than source. Pass trivially at zero upcasters.

Deliberately out of scope (recorded in ADR-090)

Outbox type-name aliasing for prematurely deleted types; producer-side rewriting of old rows.

Verification

• dotnet build MMCA.Common.slnx -c Release: 0 warnings, 0 errors (TWAE + five analyzers + PublicAPI gate; all new public symbols baselined in PublicAPI.Unshipped).
• Full suite Release: 3804 passed, 0 failed (35 new tests: registry chaining/cycle/envelope, dispatcher upcast + no-registry regression guard, broker consumer flow, startup validator, rule-level fitness tests with fixture maps, in-process end-to-end).
• FACTS regenerated (fitness methods 104 -> 106); --check clean.
• All V1/V2/V3 sample contracts live in test assemblies only, so frozen integration-event contract snapshots do not churn.

Companion docs PR: ivanball/ivanball.github.io#120 (ADR-090 + ADR-010 addendum). Additive change; release + consumer pin sweep will follow later via the normal /push-release flow (minor bump).

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https://claude.ai/code/session_016usKhixm2drfHmwmmmqwbn