add RFC 8417 golden conformance fixtures and round-trip tests

[hstern]

Summary

Adds the RFC 8417 golden conformance suite: spec-derived claims-set fixtures under testdata/ and a table-driven round-trip test (conformance_test.go) that drives each one through the full consumer/producer cycle.

RFC 8417 ships prose example figures rather than machine-readable vectors and defines no external conformance suite, so the library's conformance story is internal golden fixtures.

Fixtures

File	Source	Exercises
scim_create.json	RFC 8417 §2.1, transcribed verbatim from the SCIM provisioning example	iss, iat, jti, two-member aud array, one SCIM create event
minimal.json	Synthetic — smallest SET satisfying the §2.2 MUSTs	one event with the empty-object {} payload
txn_toe_subid.json	Synthetic	txn, toe distinct from iat, an RFC 9493 iss_sub sub_id, aud array
single_aud.json	Synthetic	bare-string aud, one event decoded to a typed value via the registry

Synthetic fixtures use RFC 2606 example domains and illustrative (non-IANA) event-type URIs; provenance is documented in testdata/README.md. No non-spec vocabulary is presented as if it came from the RFC.

What the test asserts

For each fixture: Parse → assert decoded SET fields (iss/iat/jti/aud/sub_id/txn/toe/events) → Validate returns nil → typed decode through the registry (one registered URI to a typed value, the rest left raw) → Encode. It then pins byte-stability two ways:

• every event payload appears in the Encode output in its canonical compact form with member order preserved — the property json.RawMessage buys over map[string]any;
• Parse → Encode → Parse is a fixed point.

The registry decoder is registered once in init so it survives -shuffle/-count without a duplicate-registration panic.

Scope

No library code changes — fixtures and tests only. Runs under the existing test CI job; no workflow changes.

Local checks

gofmt -l clean, go vet ./..., go mod tidy -diff clean, go test -race -shuffle=on ./... green, golangci-lint 0 issues.

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