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.
`Window :: #jni_class("...") { #extends View; ... }` should parse,
today's 2.3 parser doesn't recognise `#extends` as a token and the
body loop reports "expected method name". The make-green follow-up
adds `hash_extends`/`hash_implements` lexer tokens, refactors
`JniClassDecl.methods` into a `members` tagged union, and dispatches
in the body loop on the leading token.
