refactor(ffi-linkage): Phase 9.2a — rename runtime-class TYPE names → Runtime* (Decision 5)

Mechanical, collision-free PascalCase renames (object-model axis, not linkage):
ForeignClassDecl→RuntimeClassDecl, ForeignMethodDecl→RuntimeMethodDecl,
ForeignClassMember→RuntimeClassMember, ForeignFieldDecl→RuntimeFieldDecl,
ForeignRuntime→RuntimeKind, ForeignClassPrefix→RuntimeClassPrefix. Snapshot-neutral;
suite green (646/444). Remaining 9.2: snake_case state (foreign_class_map,
current_foreign_class, foreign_path [coupled to .sx hooks], the foreign_class_decl
union variant) + the parse/lower/resolve fn names + ForeignClassDecl.is_foreign flag.

src/ir/jni_descriptor.zig

@@ -103,13 +103,13 @@ pub fn writeType(
     }
 }
 
-/// Derives the full `(args)ret` method descriptor for a `ForeignMethodDecl`.
+/// Derives the full `(args)ret` method descriptor for a `RuntimeMethodDecl`.
 /// The first param is skipped when `is_static == false` (it's the implicit
 /// `self: *Self` receiver, which doesn't appear in the JNI descriptor).
 pub fn deriveMethod(
     allocator: std.mem.Allocator,
     ctx: Context,
-    method: ast.ForeignMethodDecl,
+    method: ast.RuntimeMethodDecl,
 ) DeriveError![]u8 {
     // `#jni_method_descriptor("(Sig)Ret")` short-circuits derivation
     // entirely. Allocate a copy so the caller has uniform ownership