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.

examples/ffi-jni-class-01-empty.sx

@@ -1,5 +1,5 @@
 // Phase 2 step 2.0 (PLAN-FFI.md): xfail parser test for the
-// type-introducer `Foo :: #jni_class("java/path/Foo") { }` directive.
+// type-introducer `Foo :: #foreign #jni_class("java/path/Foo") { }` directive.
 //
 // Today's parser doesn't recognize `#jni_class` as a known hash
 // directive after `::`, so it falls through to expression parsing
@@ -10,7 +10,7 @@
 
 #import "modules/std.sx";
 
-Foo :: #jni_class("java/path/Foo") { }
+Foo :: #foreign #jni_class("java/path/Foo") { }
 
 main :: () -> s32 {
     print("parse-only ok\n");

examples/ffi-jni-class-02-method.sx

@@ -10,7 +10,7 @@
 
 // `Self` here refers to the enclosing `View` type — resolved at
 // 2.x sema, not at parse time.
-View :: #jni_class("android/view/View") {
+View :: #foreign #jni_class("android/view/View") {
     getId :: (self: *Self) -> s32;
 }
 

examples/ffi-jni-class-03-static.sx

@@ -9,7 +9,7 @@
 
 #import "modules/std.sx";
 
-Math :: #jni_class("java/lang/Math") {
+Math :: #foreign #jni_class("java/lang/Math") {
     static abs :: (n: s32) -> s32;
 }
 

examples/ffi-jni-class-04-extends.sx

@@ -12,9 +12,9 @@
 
 #import "modules/std.sx";
 
-View :: #jni_class("android/view/View") { }
+View :: #foreign #jni_class("android/view/View") { }
 
-Window :: #jni_class("android/view/Window") {
+Window :: #foreign #jni_class("android/view/Window") {
     #extends View;
     getDecorView :: (self: *Self) -> *View;
 }

examples/ffi-jni-class-05-field.sx

@@ -8,7 +8,7 @@
 
 #import "modules/std.sx";
 
-Point :: #jni_class("android/graphics/Point") {
+Point :: #foreign #jni_class("android/graphics/Point") {
     x: s32;
     y: s32;
 }

examples/ffi-jni-class-06-desc.sx

@@ -10,7 +10,7 @@
 
 #import "modules/std.sx";
 
-Foo :: #jni_class("com/example/Foo") {
+Foo :: #foreign #jni_class("com/example/Foo") {
     weirdMethod :: (self: *Self) -> s32 #jni_method_descriptor("()I");
 }
 

examples/ffi-jni-class-07-all-runtimes.sx

@@ -17,27 +17,27 @@
 
 #import "modules/std.sx";
 
-IFoo :: #jni_interface("com/example/IFoo") {
+IFoo :: #foreign #jni_interface("com/example/IFoo") {
     bar :: (self: *Self) -> s32;
 }
 
-NSString :: #objc_class("NSString") {
+NSString :: #foreign #objc_class("NSString") {
     length :: (self: *Self) -> s32;
 }
 
-NSCopying :: #objc_protocol("NSCopying") {
+NSCopying :: #foreign #objc_protocol("NSCopying") {
     copy :: (self: *Self) -> *Self;
 }
 
-URL :: #swift_class("Foundation.URL") {
+URL :: #foreign #swift_class("Foundation.URL") {
     absoluteString :: (self: *Self) -> *void;
 }
 
-Date :: #swift_struct("Foundation.Date") {
+Date :: #foreign #swift_struct("Foundation.Date") {
     timeIntervalSince1970 :: (self: *Self) -> f64;
 }
 
-Hashable :: #swift_protocol("Swift.Hashable") {
+Hashable :: #foreign #swift_protocol("Swift.Hashable") {
     hash :: (self: *Self) -> s32;
 }
 

examples/ffi-jni-class-08-call.sx

@@ -8,7 +8,7 @@
 
 #import "modules/std.sx";
 
-Activity :: #jni_class("android/app/Activity") {
+Activity :: #foreign #jni_class("android/app/Activity") {
     getWindow :: (self: *Self) -> *void;
 }