ffi 2.9: cross-class *Foo resolves via ForeignClassDecl registry

`Context` gains an optional `classes: ?*const ClassRegistry` lookup
(sx alias → foreign path). `writeType`'s pointer-type arm now treats
`*Self` and `*Foo` uniformly: `Self` resolves to `enclosing_path`,
any other named target is looked up in the registry. Missing
registry or missing key both surface as `UnknownClassAlias`.

`DeriveError.CrossClassRefNotYetSupported` retired in favour of
`UnknownClassAlias` — the new error fires for both
"no-registry-provided" and "alias-not-in-registry", giving the
caller (later, sema with a real diagnostic) one error variant to
handle.

Four new unit tests: cross-class resolves with registry, errors
without registry, errors with empty registry, and end-to-end
`deriveMethod` with chained `*Self`/`*Foo` (`getDecorView ::
(self: *Self) -> *View → ()Landroid/view/View;`).

13 jni_descriptor tests pass; 126/126 examples still green.

src/ir/jni_descriptor.test.zig

@@ -99,7 +99,30 @@ test "slice of primitive is array descriptor" {
     try std.testing.expectEqualStrings("[I", buf.items);
 }
 
-test "cross-class *Foo is not yet supported (2.9)" {
+test "cross-class *Foo resolves via class registry" {
+    const a = std.testing.allocator;
+    var arena = std.heap.ArenaAllocator.init(a);
+    defer arena.deinit();
+    const aa = arena.allocator();
+
+    var registry = desc.ClassRegistry.init(a);
+    defer registry.deinit();
+    try registry.put("Window", "android/view/Window");
+    try registry.put("View", "android/view/View");
+
+    const foo = try makeTypeExpr(aa, "Window");
+    const ptr = try makePointer(aa, foo);
+
+    var buf: std.ArrayList(u8) = .empty;
+    defer buf.deinit(a);
+    try desc.writeType(a, &buf, .{
+        .enclosing_path = "android/view/View",
+        .classes = &registry,
+    }, ptr);
+    try std.testing.expectEqualStrings("Landroid/view/Window;", buf.items);
+}
+
+test "cross-class *Foo without registry errors" {
     const a = std.testing.allocator;
     var arena = std.heap.ArenaAllocator.init(a);
     defer arena.deinit();
@@ -110,8 +133,64 @@ test "cross-class *Foo is not yet supported (2.9)" {
 
     var buf: std.ArrayList(u8) = .empty;
     defer buf.deinit(a);
-    const result = desc.writeType(a, &buf, .{ .enclosing_path = "android/view/View" }, ptr);
-    try std.testing.expectError(desc.DeriveError.CrossClassRefNotYetSupported, result);
+    const result = desc.writeType(a, &buf, .{
+        .enclosing_path = "android/view/View",
+    }, ptr);
+    try std.testing.expectError(desc.DeriveError.UnknownClassAlias, result);
+}
+
+test "cross-class *Foo with empty registry errors" {
+    const a = std.testing.allocator;
+    var arena = std.heap.ArenaAllocator.init(a);
+    defer arena.deinit();
+    const aa = arena.allocator();
+
+    var registry = desc.ClassRegistry.init(a);
+    defer registry.deinit();
+
+    const foo = try makeTypeExpr(aa, "WindowInsets");
+    const ptr = try makePointer(aa, foo);
+
+    var buf: std.ArrayList(u8) = .empty;
+    defer buf.deinit(a);
+    const result = desc.writeType(a, &buf, .{
+        .enclosing_path = "android/view/Window",
+        .classes = &registry,
+    }, ptr);
+    try std.testing.expectError(desc.DeriveError.UnknownClassAlias, result);
+}
+
+test "deriveMethod chains *Foo returns and params" {
+    const a = std.testing.allocator;
+    var arena = std.heap.ArenaAllocator.init(a);
+    defer arena.deinit();
+    const aa = arena.allocator();
+
+    var registry = desc.ClassRegistry.init(a);
+    defer registry.deinit();
+    try registry.put("Window", "android/view/Window");
+    try registry.put("View", "android/view/View");
+    try registry.put("WindowInsets", "android/view/WindowInsets");
+
+    // getDecorView :: (self: *Self) -> *View  →  ()Landroid/view/View;
+    const self_te = try makeTypeExpr(aa, "Self");
+    const self_ptr = try makePointer(aa, self_te);
+    const view_te = try makeTypeExpr(aa, "View");
+    const view_ptr = try makePointer(aa, view_te);
+
+    const method: ast.ForeignMethodDecl = .{
+        .name = "getDecorView",
+        .params = &.{self_ptr},
+        .param_names = &.{"self"},
+        .return_type = view_ptr,
+        .is_static = false,
+    };
+    const out = try desc.deriveMethod(a, .{
+        .enclosing_path = "android/view/Window",
+        .classes = &registry,
+    }, method);
+    defer a.free(out);
+    try std.testing.expectEqualStrings("()Landroid/view/View;", out);
 }
 
 test "deriveMethod skips implicit self for instance methods" {