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.
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