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/sdl3.sx

@@ -122,7 +122,7 @@ impl Platform for SdlPlatform {
 
         g_sdl_plat = self;
         self.last_perf = SDL_GetPerformanceCounter();
-        true;
+        true
     }
 
     run_frame_loop :: (self: *SdlPlatform, frame_fn: Closure()) {
@@ -164,7 +164,7 @@ impl Platform for SdlPlatform {
         result : []Event = ---;
         result.ptr = self.events.items;
         result.len = self.events.len;
-        result;
+        result
     }
 
     begin_frame :: (self: *SdlPlatform) -> FrameContext {
@@ -195,7 +195,7 @@ impl Platform for SdlPlatform {
             pixel_h = self.pixel_h,
             dpi_scale = self.dpi_scale,
             delta_time = self.delta_time,
-        };
+        }
     }
 
     end_frame :: (self: *SdlPlatform) {
@@ -203,11 +203,11 @@ impl Platform for SdlPlatform {
     }
 
     safe_insets :: (self: *SdlPlatform) -> EdgeInsets {
-        EdgeInsets.zero();
+        EdgeInsets.zero()
     }
 
     keyboard :: (self: *SdlPlatform) -> KeyboardState {
-        KeyboardState.zero();
+        KeyboardState.zero()
     }
 
     show_keyboard :: (self: *SdlPlatform) { }
@@ -242,7 +242,7 @@ sdl_event_watch :: (userdata: *void, event: *SDL_Event) -> bool callconv(.c) {
             }
         }
     }
-    true;
+    true
 }
 
 sdl_wasm_tick :: () callconv(.c) {