ffi 2.2 green: parser collects #jni_class instance method body items

New `JniMethodDecl` AST struct (name, params, param_names,
return_type — no body, foreign declaration). `JniClassDecl.body`
becomes `methods: []const JniMethodDecl`. parseJniClassDecl loops
over body items, parsing each `name :: (self: *Self, args...) -> Ret;`
similarly to parseProtocolDecl but requiring `;` (no body brace).

`static`, fields, `#extends`, `#implements`, and the other six
directive forms land in 2.3–2.7. Sema/lower still treat the decl
as an opaque type alias — descriptor derivation arrives in 2.8+.

121/121 examples green.

tests/expected/ffi-jni-class-02-method.exit

@@ -1 +1 @@
-1
+0

tests/expected/ffi-jni-class-02-method.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/ffi-jni-class-02-method.sx:14:5: error: non-empty `#jni_class` body not yet supported (Phase 2.1 accepts only the empty/opaque form)
+parse-only ok