fix: restore two schema checks dropped by #64's conflict resolution

[macanderson]

Follow-up to #64. main is currently serving a stale schema, and the check that would catch it was deleted in the same merge.

What happened

#64's conflict resolution kept its own schema/validate-examples.py wholesale, discarding both checks #63 had added to that file on main:

Check	Added by	Purpose
reference-serialized vector validation	#63 (issue #54)	Validates the Rust serializer's own minimal output against the schema — the half curated examples cannot catch
$id + served-copy byte-identity	#63	Asserts $id names the live domain and site/public/schema/ matches the source

Neither loss was visible in CI. Both are additive validations — deleting them removes coverage without failing anything. #64 went green with them gone, and main is green right now.

This is not theoretical

#64 rewrote the ContextQuery $comment. The served copy under site/public/schema/ was never refreshed, so on main today:

$ diff schema/contextgraph-envelope.schema.json site/public/schema/contextgraph-envelope.schema.json
*** DRIFTED ***

That is exactly the case #63's comment describes — "a stale schema that still resolves is one that silently validates the wrong thing", worse than a 404. The merge introduced the drift and deleted the detector in one move.

The $comment collision

Worth flagging: #63 and #64 fixed the ContextQuery.required bug independently and identically. Both removed kinds/anchors; only the $comment prose differed. This PR takes #63's wording, which is the better text — it names the conformance suite's own sample_query() and records that absence and [] mean the same thing. schema/ is byte-identical to the served copy again.

The CHANGELOG bullet from #64 claiming that schema fix is reworded to claim only what #64 actually contributed uniquely: the regression test and the cross-audit.

Verification

Both restored checks are mutation-tested, not assumed:

# $id check, against main's real current state:
FAIL  the served schema copy is byte-identical to the source

# vector check, with ContextQuery.required reverted to the pre-#63 list:
FAIL  reference-vectors.ndjson line 4

Gate	Result
python3 schema/validate-examples.py	39 checks pass (37 on main — the two dropped ones restored)
cargo test --workspace	292 passed, 0 failed
cargo fmt --all --check	clean
cargo clippy --workspace --all-targets -- -D warnings	clean

Review notes

• No behavioral change. This restores deleted validation and resyncs a served file. No wire type, schema constraint, or conformance rule changes.
• Section numbering reconciled: 1–2 examples/, 3 reference vectors, 4 SPEC.md, 5 schema identity.
• The broader lesson worth capturing: this bug class — a merge that deletes an additive check — is invisible to CI by construction. Green CI on a conflict resolution is not evidence the resolution was clean. Diffing the merge result against both parents is what surfaces it.

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