ffi 2.7 green: parser accepts all seven type-introducer directive forms

Six new lexer tokens (`hash_jni_interface`, `hash_objc_class`,
`hash_objc_protocol`, `hash_swift_class`, `hash_swift_struct`,
`hash_swift_protocol`) join the existing `hash_jni_class`. All seven
share the body grammar from Phases 2.1–2.6.

AST refactored: `JniClassDecl` → `ForeignClassDecl` with a
`runtime: ForeignRuntime` enum discriminator; `JniMethodDecl` →
`ForeignMethodDecl` (with `jni_descriptor_override` renamed for
clarity since it's JNI-only); `JniFieldDecl` → `ForeignFieldDecl`;
`JniClassMember` → `ForeignClassMember`. AST variant renamed
`jni_class_decl` → `foreign_class_decl`.

`parseForeignClassDecl` takes the runtime as a parameter; the
`parseConstBinding` dispatch table now maps each of the seven
directive tokens to its `ForeignRuntime` variant via
`foreignRuntimeForCurrent`. No codegen yet — Phase 3 picks up Obj-C
runtime, Phase 4 picks up Swift. Runtime-specific body items (fields,
descriptor override) are validated at sema time in later steps.

126/126 examples green.

src/lexer.zig

@@ -83,6 +83,12 @@ pub const Lexer = struct {
                 .{ "#jni_call", Tag.hash_jni_call },
                 .{ "#jni_static_call", Tag.hash_jni_static_call },
                 .{ "#jni_class", Tag.hash_jni_class },
+                .{ "#jni_interface", Tag.hash_jni_interface },
+                .{ "#objc_class", Tag.hash_objc_class },
+                .{ "#objc_protocol", Tag.hash_objc_protocol },
+                .{ "#swift_class", Tag.hash_swift_class },
+                .{ "#swift_struct", Tag.hash_swift_struct },
+                .{ "#swift_protocol", Tag.hash_swift_protocol },
                 .{ "#extends", Tag.hash_extends },
                 .{ "#implements", Tag.hash_implements },
                 .{ "#jni_method_descriptor", Tag.hash_jni_method_descriptor },