ffi 2.4 green: #extends and #implements body items

Two new lexer tokens `hash_extends` / `hash_implements` (global tokens,
context-meaningful inside #jni_class bodies — same pattern as #using).
`JniClassDecl.methods` refactored into `members: []const JniClassMember`,
a tagged union with `method` / `extends` / `implements` variants.
Body loop dispatches on the leading token: `#extends Alias;` /
`#implements Alias;` consume the alias name and push a non-method
member; everything else falls through to the existing method path.

The alias on the right of `#extends` is the sx-side name (resolved
to the corresponding #jni_class at sema time in a later step), not
the foreign Java path — the path lives only in the alias's own
directive arg.

123/123 examples green.

tests/expected/ffi-jni-class-04-extends.exit

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

tests/expected/ffi-jni-class-04-extends.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/ffi-jni-class-04-extends.sx:18:5: error: expected method name in '#jni_class' body
+parse-only ok