fix(managed): reject non-method signature headers and invalid TypeRef scopes

The method signature header reader now owns the calling-convention rule
that only FNPTR applied: FIELD, LOCAL_SIG, PROPERTY, and GENERICINST
headers, the reserved bit, and EXPLICITTHIS without HASTHIS are malformed.
A MethodSpec whose method has a PROPERTY header is no longer accepted as a
generic method, and member signatures with such headers no longer decode
as methods.

The type-name lookup validates a TypeRef's ResolutionScope coded index and
reports an out-of-range scope as invalid-row, keeping the readable name.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/managed-code-analysis.md

@@ -105,9 +105,9 @@ generic method it instantiates. A field access is named only by a field or a
 field-signature MemberRef. TypeSpec and MethodSpec blobs are decoded with the
 member-signature grammar, and a MethodSpec must instantiate a generic method
 with as many type arguments as it declares. Malformed or unsupported rows and
-blobs, and arity mismatches, make coverage partial; a readable MethodSpec
-method column keeps its name. Older member Evidence without these fields reads
-them as `null`: unknown.
+blobs, and arity mismatches, make coverage partial. A readable MethodSpec
+method column, or a TypeRef whose resolution scope is invalid, keeps its name.
+Older member Evidence without these fields reads them as `null`: unknown.
 
 ## Analysis objective