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.

library/modules/platform/android_jni.sx

@@ -11,21 +11,21 @@
 // signatures (`getWindow :: (self: *Self) -> *Window`) still resolve
 // against the bare name within the namespace.
 
-WindowInsets :: #jni_class("android/view/WindowInsets") {
+WindowInsets :: #foreign #jni_class("android/view/WindowInsets") {
     getSystemWindowInsetTop    :: (self: *Self) -> s32;
     getSystemWindowInsetLeft   :: (self: *Self) -> s32;
     getSystemWindowInsetBottom :: (self: *Self) -> s32;
     getSystemWindowInsetRight  :: (self: *Self) -> s32;
 }
 
-View :: #jni_class("android/view/View") {
+View :: #foreign #jni_class("android/view/View") {
     getRootWindowInsets :: (self: *Self) -> *WindowInsets;
 }
 
-Window :: #jni_class("android/view/Window") {
+Window :: #foreign #jni_class("android/view/Window") {
     getDecorView :: (self: *Self) -> *View;
 }
 
-Activity :: #jni_class("android/app/Activity") {
+Activity :: #foreign #jni_class("android/app/Activity") {
     getWindow :: (self: *Self) -> *Window;
 }