ffi 2.6 green: #jni_method_descriptor("(Sig)Ret") override

New `hash_jni_method_descriptor` lexer token + LSP keyword
classification. `JniMethodDecl` gains `desc_override: ?[]const u8`.
parseJniClassDecl accepts an optional `#jni_method_descriptor("...")`
clause between the return type and the terminating `;`, stashing the
literal as the override. Auto-derivation in Phase 2.8 will treat
this as the precedence override when present.

The 2.6 xfail commit (0ed4799) used the working name `#desc` in its
test file; this commit renames to `#jni_method_descriptor` for
parallel naming with the rest of the FFI directive set (`#jni_call`,
`#jni_class`, `#jni_env`, ...). Test snapshot flips xfail → green.

125/125 examples green.

src/ast.zig

@@ -539,6 +539,7 @@ pub const JniMethodDecl = struct {
     param_names: []const []const u8,
     return_type: ?*Node, // null = void
     is_static: bool = false, // true for `static name :: ...`
+    desc_override: ?[]const u8 = null, // `#desc("(Sig)Ret")` — skips auto-derivation in 2.8+
 };
 
 pub const JniFieldDecl = struct {