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/lexer.zig

@@ -85,6 +85,7 @@ pub const Lexer = struct {
                 .{ "#jni_class", Tag.hash_jni_class },
                 .{ "#extends", Tag.hash_extends },
                 .{ "#implements", Tag.hash_implements },
+                .{ "#jni_method_descriptor", Tag.hash_jni_method_descriptor },
             };
             inline for (directives) |d| {
                 const keyword = d[0];