fix(#417): codex round 99 — full C# identifier categories in type scan

IsLetterOrDigit misses identifier continuation characters the language
permits as LITERALS — letter numbers, connector punctuation, combining
marks, and format characters — so a tuple element name carrying a
decomposed combining mark stopped TryConsumeTypeParen and left the
hole identifier-led while the interpolated null bridged the fragments.
IsCSharpIdentifierChar now implements the language's identifier-part
categories and replaces IsLetterOrDigit in the type scanner's
whitelist.

The atom checks' keyword walk deliberately keeps the narrow class:
keywords are pure ASCII and an escape- or combining-spelled lookalike
is an IDENTIFIER by the spec, so passing it as identifier-led is
correct, not a gap.

Mutation-verified: a tuple element name with a literal decomposed
combining mark (byte-verified present in the injected file) and one
with connector punctuation each failed PostgresImagePin with the
static-atom refusal; a hole reading a combining-mark runtime
identifier passed. Restored, rebuilt, all three SchemaDocsTests
green.

Author: mforce

tests/Cluckwork.Api.IntegrationTests/SchemaDocsTests.cs

@@ -1141,6 +1141,20 @@ private static int SkipTrivia(string t, int p)
     // matching close paren, or -1 when the interior carries anything else
     // (an operator, a quote): that is an expression, not a type. A `)` or
     // `>` that closes what is not open is malformed and also -1.
+    // C# identifier characters, per the language's identifier-part
+    // categories — IsLetterOrDigit alone misses letter numbers, connector
+    // punctuation, combining marks, and format characters, all of which are
+    // legal in identifiers (and therefore in type names and tuple element
+    // names) as LITERAL characters, not only as \u escapes.
+    private static bool IsCSharpIdentifierChar(char c) =>
+        char.IsLetterOrDigit(c)
+        || c == '_'
+        || char.GetUnicodeCategory(c) is System.Globalization.UnicodeCategory.LetterNumber
+            or System.Globalization.UnicodeCategory.ConnectorPunctuation
+            or System.Globalization.UnicodeCategory.NonSpacingMark
+            or System.Globalization.UnicodeCategory.SpacingCombiningMark
+            or System.Globalization.UnicodeCategory.Format;
+
     private static int TryConsumeTypeParen(string t, int p)
     {
         var parenDepth = 1;
@@ -1183,7 +1197,7 @@ static bool Hex(string s, int start, int count)
                 if (p + 1 < t.Length && t[p + 1] == 'U' && Hex(t, p + 2, 8)) { p += 10; continue; }
                 return -1;
             }
-            else if (!(char.IsLetterOrDigit(c) || c is '_' or '.' or ':' or ',' or '?' or '[' or ']' or '*' or '@'))
+            else if (!(IsCSharpIdentifierChar(c) || c is '.' or ':' or ',' or '?' or '[' or ']' or '*' or '@'))
                 return -1;
             p++;
         }