ffi: define-by-default #jni_class + #foreign modifier + #jni_main token

Flip the surface semantics for type-introducer directives: bare
`Foo :: #jni_class("path") { ... }` now means "DEFINE a new Java class
at that path" (sx-side provides the implementations). The `#foreign`
prefix modifier flips it back to "REFERENCE an existing class on the
foreign runtime." Matches how `#foreign` already reads in sx for C
function declarations (`printf :: ... #foreign;`).

    Foo :: #foreign  #jni_class("path/to/Foo") { ... }  // reference
    Foo ::           #jni_class("path/to/Foo") { ... }  // define
    Foo :: #jni_main #jni_class("path/to/Foo") { ... }  // define + main Activity

Compiler-side changes:
- New `hash_jni_main` lexer token (the launchable-Activity marker).
  Existing `hash_foreign` is reused; no new modifier token there.
- `ForeignClassDecl` gains `is_foreign: bool` + `is_main: bool`.
  `ForeignMethodDecl` gains `body: ?*Node` so defined-class methods
  can carry sx-side implementations (foreign-class methods stay
  `;`-terminated).
- Parser learns `tryParseForeignClassPrefix` — peek-and-consume the
  modifier tokens, then dispatch to the unchanged
  `parseForeignClassDecl` with the flags threaded through.
- Sema rejects two illegal combinations: `#foreign + #jni_main`
  (can't be both an external reference and the app's main entry),
  and bodied methods on `#foreign` decls (foreign methods are
  runtime-provided).
- Lower's foreign-class dispatch errors on non-foreign decls with
  a pointer to the runtime-synthesis follow-up; defined-class
  codegen (Java class emission, RegisterNatives wiring, manifest
  entry generation) lands in a separate session.

Migration:
- `library/modules/platform/android_jni.sx`: all four foreign class
  decls (`Activity`, `Window`, `View`, `WindowInsets`) gain `#foreign`.
- `examples/ffi-jni-class-{01..08}*.sx`: every test's `#jni_class` /
  `#jni_interface` / `#objc_class` / `#objc_protocol` / `#swift_class`
  / `#swift_struct` / `#swift_protocol` usage gains `#foreign`. All
  9 files mechanical perl rename; snapshots unchanged.

Verified locally:
- `zig build test` clean.
- `bash tests/run_examples.sh` 129/129.
- `bash tests/cross_compile.sh` 3/3.
- Chess APK rebuilds, reinstalls, launches on Pixel; safe-area
  clearance preserved.

src/sema.zig

@@ -909,6 +909,25 @@ pub const Analyzer = struct {
             },
             .foreign_class_decl => |fd| {
                 try self.addSymbol(fd.name, .type_alias, null, node.span);
+                if (fd.is_foreign and fd.is_main) {
+                    try self.diagnostics.append(self.allocator, .{
+                        .level = .err,
+                        .message = "'#foreign' and '#jni_main' / '#objc_main' are mutually exclusive — a foreign-referenced class can't be the app's main entry",
+                        .span = node.span,
+                    });
+                }
+                if (fd.is_foreign) {
+                    for (fd.members) |m| switch (m) {
+                        .method => |md| if (md.body != null) {
+                            try self.diagnostics.append(self.allocator, .{
+                                .level = .err,
+                                .message = "methods on a '#foreign' class can't have bodies — they reference foreign-runtime implementations",
+                                .span = node.span,
+                            });
+                        },
+                        else => {},
+                    };
+                }
             },
             .jni_env_block => |eb| {
                 try self.analyzeNode(eb.env);