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

Ports the three deferred validation passes from the TS reference into the C#
loader (ValidationPasses), clearing all 10 deferred error fixtures from the C#
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 MetaDataLoader.Load after the FR-016 physical-name pass; each mirrors
its TS counterpart's logic + error-code/envelope contract (collect-not-throw, C#
idiom). WARN_READONLY_VALUE_OBJECT added to WarningCodes. Unlike Java, the C#
model preserves an override field's authored JsonSource, so
error-field-readonly-downgrade emits the byte-identical envelope and passes.

C# conformance: 550/550 passed, ledger cleared.

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

Author: dmealing

server/csharp/MetaObjects.Conformance.Tests/conformance-expected-failures.json

@@ -1,17 +1,6 @@
 {
   "language": "csharp",
   "_comment": "FR5a envelope-shape gates: TS is the reference port for source.format / source.files / source.jsonPath. C# is now reconciled with TS on the JSON-side FR5a envelopes (parser malformed-JSON catch builds a $-rooted JsonSource with the source-file name; inline-attr loop reports ERR_RESERVED_ATTR / ERR_UNKNOWN_ATTR / ERR_BAD_ATTR_VALUE at the parent's path — not key-deeper). Keep this file as the documented ledger surface.",
-  "_notes": "FR5c (finalized 2026-05-27, all four ports): merge-phase contributors + ERR_MERGE_CONFLICT envelope + WARN_DUPLICATE_DECLARATION. FR5d (finalized 2026-05-27, all four ports): reference-resolution errors emit format=resolved with referrer + target. FR-013 (@readOnly), FR-014 (TPH discriminator), and FR-015 (@parameterRef) attrs are now registered in the C# registry vocabulary (SP-G registry-conformance), so positive round-trip fixtures pass; their VALIDATION passes have not yet shipped in the C# port, so the error-* fixtures remain deferred until each per-port fan-out lands.",
-  "fixtures": [
-    "error-field-readonly-assigned-primary",
-    "error-field-readonly-downgrade",
-    "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"
-  ]
+  "_notes": "FR5c (finalized 2026-05-27, all four ports): merge-phase contributors + ERR_MERGE_CONFLICT envelope + WARN_DUPLICATE_DECLARATION. FR5d (finalized 2026-05-27, all four ports): reference-resolution errors emit format=resolved with referrer + target. FR-013 (@readOnly), FR-014 (TPH discriminator), and FR-015 (@parameterRef) VALIDATION passes shipped in the C# port (ValidationPasses.ValidateFieldReadOnly / ValidateDiscriminator / ValidateSourceParameterRef); all error-* fixtures now produce the expected envelopes. No interim known-gaps.",
+  "fixtures": []
 }

server/csharp/MetaObjects/Errors.cs

@@ -96,6 +96,13 @@ public static class WarningCodes
     /// serializer rewrites the attr key to the kind-matching alias.
     /// </summary>
     public const string WARN_LEGACY_PHYSICAL_NAME_ALIAS = "WARN_LEGACY_PHYSICAL_NAME_ALIAS";
+
+    /// <summary>
+    /// FR-013: <c>@readOnly: true</c> on a field child of an <c>object.value</c>.
+    /// Value-objects have no persistence semantics, so the read-only contract is
+    /// advisory (codegen may use it for record/struct treatment).
+    /// </summary>
+    public const string WARN_READONLY_VALUE_OBJECT = "WARN_READONLY_VALUE_OBJECT";
 }
 
 /// <summary>

server/csharp/MetaObjects/Loader/MetaDataLoader.cs

@@ -377,6 +377,24 @@ public LoadResult Load(IReadOnlyList<IMetaDataSource> sources)
                 }
             }
 
+            // FR-013: field-level @readOnly cross-attribute rules.
+            var roResult = ValidationPasses.ValidateFieldReadOnly(root);
+            errors.AddRange(roResult.Errors);
+            if (roResult.Warnings.Count > 0)
+            {
+                envelopeWarnings.AddRange(roResult.Warnings);
+                foreach (var w in roResult.Warnings)
+                {
+                    warnings.Add(w.Message);
+                }
+            }
+
+            // FR-014: TPH discriminator cross-attribute rules.
+            errors.AddRange(ValidationPasses.ValidateDiscriminator(root));
+
+            // FR-015: source.rdb @parameterRef typed-input rules.
+            errors.AddRange(ValidationPasses.ValidateSourceParameterRef(root));
+
             // Pass 14 (FR-017): M:N relationship slim-vocabulary validation —
             // symmetric-self-join-only / symmetric⊕sourceRefField (ERR_BAD_ATTR_VALUE);
             // junction-two-references / sourceRefField-match / M:N-attr-on-1:N