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).

examples/1601-platform-sdl-graphics.sx

@@ -8,7 +8,7 @@ WIDTH  :f32: 800;
 HEIGHT :f32: 600;
 
 vec4 :: (x: f32, y: f32, z: f32, w: f32) -> Vector(4, f32) {
-    .[x, y, z, w];
+    .[x, y, z, w]
 }
 
 // ---- Matrix44: column-major 4×4 matrix ----
@@ -24,7 +24,7 @@ mat4_multiply :: (a: *Matrix44, b: *Matrix44) -> Matrix44 {
     out.c1 = a.c0 * b.c1.x + a.c1 * b.c1.y + a.c2 * b.c1.z + a.c3 * b.c1.w;
     out.c2 = a.c0 * b.c2.x + a.c1 * b.c2.y + a.c2 * b.c2.z + a.c3 * b.c2.w;
     out.c3 = a.c0 * b.c3.x + a.c1 * b.c3.y + a.c2 * b.c3.z + a.c3 * b.c3.w;
-    out;
+    out
 }
 multiply :: ufcs mat4_multiply;
 
@@ -36,7 +36,7 @@ mat4_perspective :: (fov: f32, aspect: f32, near: f32, far: f32) -> Matrix44 {
     m.c1 = vec4(0.0, f, 0.0, 0.0);
     m.c2 = vec4(0.0, 0.0, (far + near) / (near - far), -1.0);
     m.c3 = vec4(0.0, 0.0, (2.0 * far * near) / (near - far), 0.0);
-    m;
+    m
 }
 
 mat4_rotate_y :: (angle: f32) -> Matrix44 {
@@ -47,7 +47,7 @@ mat4_rotate_y :: (angle: f32) -> Matrix44 {
     m.c1 = vec4(0.0, 1.0, 0.0, 0.0);
     m.c2 = vec4(s, 0.0, c, 0.0);
     m.c3 = vec4(0.0, 0.0, 0.0, 1.0);
-    m;
+    m
 }
 
 mat4_rotate_x :: (angle: f32) -> Matrix44 {
@@ -58,7 +58,7 @@ mat4_rotate_x :: (angle: f32) -> Matrix44 {
     m.c1 = vec4(0.0, c, s, 0.0);
     m.c2 = vec4(0.0, 0.0 - s, c, 0.0);
     m.c3 = vec4(0.0, 0.0, 0.0, 1.0);
-    m;
+    m
 }
 
 mat4_translate :: (tx: f32, ty: f32, tz: f32) -> Matrix44 {
@@ -67,7 +67,7 @@ mat4_translate :: (tx: f32, ty: f32, tz: f32) -> Matrix44 {
     m.c1 = vec4(0.0, 1.0, 0.0, 0.0);
     m.c2 = vec4(0.0, 0.0, 1.0, 0.0);
     m.c3 = vec4(tx, ty, tz, 1.0);
-    m;
+    m
 }
 
 // ---- Main ----