chore(csharp): pre-merge simplifier pass — FR5a polish

Two small, behavior-preserving tidies to the FR5a C# port:

1. JsonPath.cs — dedupe the two identical compiled identifier regex
   instances (one in JsonPathBuilder, one in JsonPath) into a single
   internal JsonPathRegex.Ident. Removes a copy and makes intent
   obvious (one rule, two consumers).

2. ValidationPasses.cs — remove unused default-null defaults from
   three private helpers (ValidateFromPath label + originSource,
   ValidateViaPath originSource, CheckFilterClauses layoutSource).
   Every call site already passes these arguments; the defaults
   were dead and let a future bug elide a source unnoticed. Now
   the type system requires them.

No public API changes. C# all 4 projects 561/561 still green
(Conformance 302, Render 88, Codegen 164, Cli 7).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dmealing

server/csharp/MetaObjects/Loader/ValidationPasses.cs

@@ -314,8 +314,8 @@ private static void ValidateFromPath(
         string projectionName,
         string fieldName,
         List<MetaError> errors,
-        string label = "origin.passthrough.@from",
-        ErrorSource? originSource = null)
+        string label,
+        ErrorSource originSource)
     {
         int dotIdx = fromAttr.IndexOf('.', StringComparison.Ordinal);
         if (dotIdx < 1 || dotIdx == fromAttr.Length - 1)
@@ -363,7 +363,7 @@ private static void ValidateViaPath(
         string projectionName,
         string fieldName,
         List<MetaError> errors,
-        ErrorSource? originSource = null)
+        ErrorSource originSource)
     {
         var segments = viaAttr.Split('.');
         if (segments.Length < 2)
@@ -502,7 +502,7 @@ private static void CheckFilterClauses(
         string entityName,
         string layoutName,
         List<MetaError> errors,
-        ErrorSource? layoutSource = null)
+        ErrorSource layoutSource)
     {
         foreach (var (key, clause) in filter)
         {

server/csharp/MetaObjects/Source/JsonPath.cs

@@ -13,6 +13,16 @@
 
 namespace MetaObjects.Source;
 
+/// <summary>
+/// Shared identifier regex for dot-vs-bracket key dispatch. Used by both
+/// <see cref="JsonPathBuilder"/> and the <see cref="JsonPath"/> static helpers.
+/// </summary>
+internal static class JsonPathRegex
+{
+    public static readonly Regex Ident =
+        new("^[A-Za-z_][A-Za-z0-9_]*$", RegexOptions.Compiled);
+}
+
 /// <summary>
 /// Builds the canonical JSONPath string for a node as the parser walks the
 /// JSON tree. Push a key or index when descending; pop when returning.
@@ -24,9 +34,6 @@ namespace MetaObjects.Source;
 /// </summary>
 public sealed class JsonPathBuilder
 {
-    private static readonly Regex IdentRe =
-        new("^[A-Za-z_][A-Za-z0-9_]*$", RegexOptions.Compiled);
-
     private readonly List<Segment> _segments = new();
 
     /// <summary>Push an object key segment (e.g. <c>.foo</c> or <c>['my-key']</c>).</summary>
@@ -61,7 +68,7 @@ public override string ToString()
             else
             {
                 string key = seg.Key!;
-                if (IdentRe.IsMatch(key))
+                if (JsonPathRegex.Ident.IsMatch(key))
                 {
                     sb.Append('.').Append(key);
                 }
@@ -84,16 +91,13 @@ private enum SegmentKind { Key, Index }
 /// </summary>
 public static class JsonPath
 {
-    private static readonly Regex IdentRe =
-        new("^[A-Za-z_][A-Za-z0-9_]*$", RegexOptions.Compiled);
-
     /// <summary>
     /// Render a single object-key segment as it would appear in canonical form,
     /// without the leading <c>$</c> — i.e. <c>.foo</c> or <c>['my-key']</c>.
     /// </summary>
     public static string SegmentForKey(string key)
     {
-        return IdentRe.IsMatch(key)
+        return JsonPathRegex.Ident.IsMatch(key)
             ? $".{key}"
             : $"['{key.Replace("'", "\\'")}']";
     }