feat(lang): block value requires no trailing ; (Rust-style)

A block's value is now its last statement ONLY when that statement is a
trailing expression with no `;`. A trailing `;` discards the value,
leaving the block void. This makes value-vs-statement explicit and lets
the compiler reject "this block was supposed to produce a value".

Compiler:
- Parser records `Block.produces_value` (last stmt is a no-`;` trailing
  expression) + `Block.discarded_semi` (the `;` that discarded a value),
  via `expectSemicolonAfter`. A trailing expression before `}` may now
  omit its `;` (previously a parse error). Match-arm and else-arm bodies
  are built value-producing regardless of the arm `;` (arms are exempt —
  the `;` is an arm terminator).
- Lowering: `lowerBlockValue` / the block-expr path / `inferExprType`
  respect `produces_value`. A value-position block that discards its value
  is a hard error (`lowerValueBody` for function bodies; the value-context
  `.block` path for if/else branches, `catch` bodies, value bindings,
  match arms). Pure-failable `-> !` bodies (value rides the error channel)
  and a value-if whose branches are void are handled without false errors.
- `defer`/`onfail` cleanup bodies lower as statements (void), so a
  trailing `;` there is fine.

Migration (behavior-preserving — output unchanged):
- stdlib + ~210 examples: dropped the trailing `;` on value-position last
  expressions. `format` now ends with an explicit `#insert "return
  result;"` (it relied on `#insert`-as-block-value, which `;` discards).
- Two `main :: () -> s32` examples that relied on the old silent
  default-return got an explicit trailing `0`.
- Rejection snapshots 0412 / 1013 regenerated (their quoted source lines
  lost a `;`); the diagnostics themselves are unchanged.

Docs/tests: specs.md "Block values" section; examples 0040 (rules) + 0041
(rejection); 3 parser unit tests. Filed issue 0066 (pre-existing
match-arm negated-literal phi-width quirk, surfaced not caused here).

Gates: zig build, zig build test, run_examples.sh -> 343 passed,
cross_compile.sh -> 7 passed (also refreshed its stale example names).

tests/cross_compile.sh

@@ -26,28 +26,28 @@ mkdir -p "$TMP_DIR"
 # compiles cleanly for the target's NDK / SDK without needing the
 # host to actually run it.
 TUPLES=(
-    "android|examples/ffi-objc-call-10-os-gate.sx"
-    "android|examples/ffi-jni-call-02-void.sx"
+    "android|examples/1336-ffi-objc-call-10-os-gate.sx"
+    "android|examples/1401-ffi-jni-call-02-void.sx"
     # Step 1.24: verify the inverse OS gate — `inline if OS == .android
     # { #jni_call(...) }` must strip its body before lowering on iOS so
     # emit_llvm doesn't try to use libjvm symbols the iOS SDK lacks.
-    "ios-sim|examples/ffi-jni-call-02-void.sx"
+    "ios-sim|examples/1401-ffi-jni-call-02-void.sx"
     # #jni_main pipeline slice 2: an example carrying a `#jni_main
     # #jni_class(...)` decl must continue to lower + link cleanly for
     # android even without an APK build (compile-only check).
-    "android|examples/ffi-jni-main-01-emit.sx"
+    "android|examples/1423-ffi-jni-main-01-emit.sx"
     # `super.method(args)` dispatch: lowers to JNI CallNonvirtualVoidMethod
     # against the parent class (Activity by default). Compile-only check
     # — runtime correctness is verified by on-device chess deploy.
-    "android|examples/ffi-jni-main-02-super.sx"
+    "android|examples/1424-ffi-jni-main-02-super.sx"
     # `Alias.new(args)` constructor dispatch: lowers to FindClass +
     # GetMethodID("<init>") + NewObject. Compile-only — runtime via chess.
-    "android|examples/ffi-jni-main-03-ctor.sx"
+    "android|examples/1425-ffi-jni-main-03-ctor.sx"
     # Week 6: iOS-simulator branch of platform.bundle. Cross-compiles
     # against the iPhoneSimulator SDK; the post-link callback then
     # writes an `.app` with the iOS-shaped Info.plist (UIDeviceFamily,
     # LSRequiresIPhoneOS, UIApplicationSceneManifest, DTPlatformName).
-    "ios-sim|examples/121-ios-sim-bundle.sx"
+    "ios-sim|examples/1615-platform-ios-sim-bundle.sx"
 )
 
 PASS=0