add race test for concurrent event-type registry access

[hstern]

What

Adds a -race contention test for the package-level event-type registry, which is guarded by a sync.RWMutex: RegisterEventType writes under Lock, LookupEventType reads under RLock. The locking choice was previously asserted by construction but never exercised under concurrent load.

TestRegistryConcurrentAccess:

• spawns 64 goroutines that concurrently register distinct event-type URIs (indexed per goroutine, so none triggers the legitimate duplicate-registration panic);
• spawns 64 more that concurrently hammer LookupEventType and Events.Typed against those same URIs;
• releases all of them together via a sync.WaitGroup start barrier to maximize reader/writer overlap;
• after the writers join, asserts every registered URI resolves to its own working decoder that round-trips its payload and reports the matching EventTypeURI.

Under the suite's go test -race -shuffle=on this proves the RWMutex admits no data race between registrations and lookups, and that no concurrent write clobbered another's entry.

Scope

• Test-only. No change to registry behavior or any other package semantics.
• The duplicate-registration, empty-URI, and nil-decoder panic cases already have dedicated tests; this PR adds the concurrency proof those leave out — it does not duplicate them.
• The fake event fixture stays unexported and shared in-test. No new public API and no importable test-helper subpackage.

Checks

• gofmt -l . clean
• go vet ./... clean
• go mod tidy -diff no drift
• go test -race -shuffle=on ./... green
• golangci-lint run ./... — 0 issues

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