fix(csharp): pre-merge review — JsonSource length-1 invariant + null-source fallback

Two cross-port drifts surfaced by the pre-merge review of FR5a, both
worth fixing now while only two ports (TS + C#) have to stay in sync.

1. Parser.cs ParseState.CurrentSource() returned a JsonSource with an
   empty Files array when the parser was invoked without a source id
   (e.g. parsing from a string buffer in tests). That violates the FR5a
   length-1 invariant and produces an envelope shape no other port
   emits. TS reference (parser-core.ts:104-114) falls back to the
   default CodeSource in the same case. C# now matches.

2. ErrorSource.cs JsonSource.Files was typed IReadOnlyList<string> with
   a Javadoc invariant ("FR5a invariant: exactly one file") that the
   type didn't enforce. Combined with #1 the type system allowed the
   bad shape through. Added a constructor-init validator that throws
   ArgumentException when Files.Count != 1, pointing callers at
   MergedSource for multi-file provenance (FR5c).

Verification: C# all 4 projects 561/561 still green (Conformance 302
including the 26 new FR5a tests + Render 88 + Codegen 164 + Cli 7).

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

Author: dmealing

server/csharp/MetaObjects/Parser.cs

@@ -130,12 +130,15 @@ private sealed class ParseState
 
         /// <summary>
         /// Build a <see cref="JsonSource"/> envelope for the current location.
-        /// The <c>Files</c> list is a length-1 array of <see cref="Source"/>
-        /// (the source id / file path), or an empty list when none is available.
+        /// When <see cref="Source"/> is null (parser invoked without a source id,
+        /// e.g. from a string buffer in tests), fall back to <see cref="CodeSource"/>
+        /// — emitting a JsonSource with an empty file list would violate the
+        /// FR5a length-1 invariant and produce an envelope shape no other port
+        /// emits. Matches the TS reference (parser-core.ts:104-114).
         /// </summary>
         public ErrorSource CurrentSource() =>
             Source is null
-                ? new JsonSource(Array.Empty<string>(), Builder.ToString())
+                ? CodeSource.Default
                 : new JsonSource(new[] { Source }, Builder.ToString());
     }
 

server/csharp/MetaObjects/Source/ErrorSource.cs

@@ -28,6 +28,17 @@ public abstract record ErrorSource
 /// <param name="JsonPath">Canonical JSONPath string for the node within <paramref name="Files"/>[0].</param>
 public sealed record JsonSource(IReadOnlyList<string> Files, string JsonPath) : ErrorSource
 {
+    /// <summary>
+    /// FR5a invariant: exactly one file. Multi-file provenance lives on
+    /// <see cref="MergedSource"/> (FR5c). Enforced at construction so the
+    /// type can be trusted by cross-port comparison.
+    /// </summary>
+    public IReadOnlyList<string> Files { get; init; } = Files.Count == 1
+        ? Files
+        : throw new ArgumentException(
+            $"JsonSource requires exactly one file path; got {Files.Count}. " +
+            "Use MergedSource for multi-file provenance.", nameof(Files));
+
     /// <inheritdoc/>
     public override string Format => "json";
 }