chore(compiler): update SymbolSerializer TODO to new tracking issue

TODO for the FunctionType serialization gap (missing
VariadicParameterIndex / OptionalParameterCount round-trip) pointed at
#536, which is now closed. Repoint it at the new follow-up issue #552
so deferred work remains discoverable per CLAUDE.md rule 8.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: antonsynd

src/Sharpy.Compiler/Project/SymbolSerializer.cs

@@ -842,7 +842,7 @@ private static void RegisterAll()
                 ot => Serialize(ot.UnderlyingType),
                 value => new OptionalType { UnderlyingType = Deserialize(value) });
 
-            // TODO(#536): FunctionType serialization loses OptionalParameterCount and
+            // TODO(#552): FunctionType serialization loses OptionalParameterCount and
             // VariadicParameterIndex. Incremental builds reconstruct FunctionType from
             // FunctionSymbol.Parameters (where IsVariadic is cached separately), so the
             // gap is benign today, but any code that reads a FunctionType straight from