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).

library/modules/platform/uikit.sx

@@ -357,7 +357,7 @@ impl Platform for UIKitPlatform {
                 self.read_screen_scale();
             }
         }
-        true;
+        true
     }
 
     run_frame_loop :: (self: *UIKitPlatform, frame_fn: Closure()) {
@@ -380,7 +380,7 @@ impl Platform for UIKitPlatform {
         result.ptr = self.events.items;
         result.len = self.events.len;
         self.events.len = 0;
-        result;
+        result
     }
 
     begin_frame :: (self: *UIKitPlatform) -> FrameContext {
@@ -395,7 +395,7 @@ impl Platform for UIKitPlatform {
             dpi_scale = self.dpi_scale,
             delta_time = self.delta_time,
             target_present_time = self.last_target_ts,
-        };
+        }
     }
 
     end_frame :: (self: *UIKitPlatform) {
@@ -417,14 +417,14 @@ impl Platform for UIKitPlatform {
             left = self.safe_left,
             bottom = bottom,
             right = self.safe_right,
-        };
+        }
     }
 
     keyboard :: (self: *UIKitPlatform) -> KeyboardState {
         KeyboardState.{
             visible = self.keyboard_visible,
             height = self.keyboard_height,
-        };
+        }
     }
 
     show_keyboard :: (self: *UIKitPlatform) {
@@ -793,7 +793,7 @@ impl Platform for UIKitPlatform {
 // callconv(.c) so this is callable from `load_gl`'s C-conv proc-loader slot.
 ios_gl_proc :: (name: [*]u8) -> *void callconv(.c) {
     rtld_default : *void = xx (0 - 2);
-    dlsym(rtld_default, name);
+    dlsym(rtld_default, name)
 }
 
 // Read a `extern NSString * const k...` global from the loaded image. The
@@ -804,7 +804,7 @@ uikit_extern_nsstring :: (name: [*]u8) -> *void {
     p := dlsym(rtld_default, name);
     if p == null { return null; }
     pp : **void = xx p;
-    pp.*;
+    pp.*
 }
 
 // ───────────────────────────────────────────────────────────────────────────
@@ -869,12 +869,12 @@ SxGLView :: #objc_class("SxGLView") {
 
 uikit_touch_location :: (touch: *void, view: *void) -> Point {
     p := #objc_call(CGPoint)(touch, "locationInView:", view);
-    Point.{ x = xx p.x, y = xx p.y };
+    Point.{ x = xx p.x, y = xx p.y }
 }
 
 uikit_first_touch :: (touches: *void) -> *void {
     touches_set : *NSSet = xx touches;
-    touches_set.anyObject();
+    touches_set.anyObject()
 }
 
 // uikit_register_gl_view_class — deleted (M3.3). SxGLView is now