feat(java): FR-013/014/015 loader validation passes (per-port fan-out)

Ports the three deferred validation passes from the TS reference into the Java
loader (ValidationPhase), clearing 9 of the 11 deferred fixtures from the Java
conformance ledger.

- FR-013 validateFieldReadOnly — ERR_READONLY_ASSIGNED_PRIMARY,
  ERR_READONLY_DOWNGRADE, WARN_READONLY_VALUE_OBJECT.
- FR-014 validateDiscriminator — ERR_DISCRIMINATOR_FIELD_NOT_FOUND,
  _VALUE_DUPLICATE, _VALUE_MISSING, _VALUE_TYPE_MISMATCH (4-pass).
- FR-015 validateSourceParameterRef — ERR_PARAMETER_REF_ON_NON_CALLABLE_KIND
  (pre-resolution), _UNRESOLVED, _NOT_VALUE_OBJECT, _PASSTHROUGH_TYPE_MISMATCH.

Wired into ValidationPhase.run after the FR-016 physical-name pass; each mirrors
its TS counterpart's logic + error-code contract (eager-throw, Java idiom).
Own-declared field nodes are read via raw own children (getChildren(false))
rather than the merge-folded getMetaFields() view so emitted envelopes carry the
authored node's source. WARN_READONLY_VALUE_OBJECT added to ErrorMessageConstants.

Two fixtures remain deferred — BOTH manifestations of one pre-existing Java
subtype-field-override gap, orthogonal to the validation logic:
  - error-field-readonly-downgrade: validation fires the correct single
    ERR_READONLY_DOWNGRADE, but a field that OVERRIDES an inherited field gets a
    MergedSource (pointing at the base declaration) rather than the subtype's
    authored json source; Contributor carries no per-contributor jsonPath, so the
    authored path isn't recoverable post-merge (TS/Python keep the override's
    plain JsonSource).
  - field-readonly-upgrade-on-subtype: positive fixture failing on canonical
    serialization of the same subtype @readonly override (same root cause).
Both are recorded honestly in the ledger for the Java field-override
provenance/serializer reconciliation.

Java metadata conformance + full module suite green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dmealing

server/java/metadata/conformance-expected-failures.json

@@ -1,17 +1,8 @@
 {
   "language": "java",
-  "_comment": "FR-013 (@readOnly), FR-014 (TPH discriminator), and FR-015 (@parameterRef) are implemented in the TS reference but the Java port has not yet shipped them. These fixtures are deferred until each per-port fan-out lands.",
+  "_comment": "FR-013 (@readOnly), FR-014 (TPH discriminator), and FR-015 (@parameterRef) VALIDATION passes shipped in the Java port (ValidationPhase.validateFieldReadOnly / validateDiscriminator / validateSourceParameterRef). 9 of 11 fixtures now pass byte-identically. The two remaining entries are BOTH manifestations of one pre-existing Java subtype-field-override gap (orthogonal to the validation fan-out), deferred until the Java loader's field-override provenance/serializer reconciliation: (1) error-field-readonly-downgrade — the validation fires the correct ERR_READONLY_DOWNGRADE with a single error, but the envelope source for a subtype field that OVERRIDES an inherited field is a MergedSource pointing at the base declaration (children[0]) rather than the subtype's authored json source (children[1]); Java clones+stamps MergedSource on extends-overrides, and Contributor carries no per-contributor jsonPath, so the subtype's authored path is not recoverable post-merge (the TS/Python reference models keep the override's plain JsonSource). (2) field-readonly-upgrade-on-subtype — a positive fixture that fails on canonical serialization of the same subtype @readOnly override (same root cause).",
   "fixtures": [
-    "error-field-readonly-assigned-primary",
     "error-field-readonly-downgrade",
-    "field-readonly-upgrade-on-subtype",
-    "error-parameter-ref-unresolved",
-    "error-parameter-ref-not-value-object",
-    "error-parameter-ref-on-table-kind",
-    "error-parameter-ref-passthrough-type-mismatch",
-    "error-tph-discriminator-field-not-found",
-    "error-tph-discriminator-value-duplicate",
-    "error-tph-discriminator-value-missing",
-    "error-tph-discriminator-value-type-mismatch"
+    "field-readonly-upgrade-on-subtype"
   ]
 }