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/math/math.sx

@@ -9,29 +9,29 @@ cos :: (x: $T) -> T #builtin;
 floor :: (x: $T) -> T #builtin;
 
 min :: (a: $T, b: T) -> T {
-    if a < b then a else b;
+    if a < b then a else b
 }
 
 max :: (a: $T, b: T) -> T {
-    if a > b then a else b;
+    if a > b then a else b
 }
 
 clamp :: (val: $T, lo: T, hi: T) -> T {
     if val < lo then lo
     else if val > hi then hi
-    else val;
+    else val
 }
 
 abs :: (x: $T) -> T {
-    if x < 0 then 0 - x else x;
+    if x < 0 then 0 - x else x
 }
 
 lerp :: (a: f32, b: f32, t: f32) -> f32 {
-    a + (b - a) * t;
+    a + (b - a) * t
 }
 
 sign :: (x: $T) -> T {
     if x > 0 then 1
     else if x < 0 then 0 - 1
-    else 0;
+    else 0
 }

library/modules/math/matrix44.sx

@@ -10,7 +10,7 @@ Mat4 :: struct {
             0.0, 1.0, 0.0, 0.0,
             0.0, 0.0, 1.0, 0.0,
             0.0, 0.0, 0.0, 1.0
-        ]};
+        ]}
     }
 
     zero :: () -> Mat4 {
@@ -19,7 +19,7 @@ Mat4 :: struct {
             0.0, 0.0, 0.0, 0.0,
             0.0, 0.0, 0.0, 0.0,
             0.0, 0.0, 0.0, 0.0
-        ]};
+        ]}
     }
 
     mul :: (self: Mat4, b: Mat4) -> Mat4 {
@@ -39,7 +39,7 @@ Mat4 :: struct {
             }
             col += 1;
         }
-        r;
+        r
     }
 
     translate :: (x: f32, y: f32, z: f32) -> Mat4 {
@@ -47,7 +47,7 @@ Mat4 :: struct {
         m.data[12] = x;
         m.data[13] = y;
         m.data[14] = z;
-        m;
+        m
     }
 
     scale :: (x: f32, y: f32, z: f32) -> Mat4 {
@@ -56,7 +56,7 @@ Mat4 :: struct {
         m.data[5] = y;
         m.data[10] = z;
         m.data[15] = 1.0;
-        m;
+        m
     }
 
     rotate_x :: (angle: f32) -> Mat4 {
@@ -67,7 +67,7 @@ Mat4 :: struct {
         m.data[6] = s;
         m.data[9] = 0.0 - s;
         m.data[10] = c;
-        m;
+        m
     }
 
     rotate_y :: (angle: f32) -> Mat4 {
@@ -78,7 +78,7 @@ Mat4 :: struct {
         m.data[2] = 0.0 - s;
         m.data[8] = s;
         m.data[10] = c;
-        m;
+        m
     }
 
     rotate_z :: (angle: f32) -> Mat4 {
@@ -89,7 +89,7 @@ Mat4 :: struct {
         m.data[1] = s;
         m.data[4] = 0.0 - s;
         m.data[5] = c;
-        m;
+        m
     }
 
     ortho :: (left: f32, right: f32, bottom: f32, top: f32, near: f32, far: f32) -> Mat4 {
@@ -101,7 +101,7 @@ Mat4 :: struct {
         m.data[13] = 0.0 - (top + bottom) / (top - bottom);
         m.data[14] = 0.0 - (far + near) / (far - near);
         m.data[15] = 1.0;
-        m;
+        m
     }
 
     perspective :: (fov: f32, aspect: f32, near: f32, far: f32) -> Mat4 {
@@ -112,6 +112,6 @@ Mat4 :: struct {
         m.data[10] = 0.0 - (far + near) / (far - near);
         m.data[11] = 0.0 - 1.0;
         m.data[14] = 0.0 - 2.0 * far * near / (far - near);
-        m;
+        m
     }
 }

library/modules/math/vector2.sx

@@ -1,26 +1,26 @@
 Vec2 :: struct {
     x, y: f32;
 
-    zero :: () -> Vec2 { Vec2.{ x = 0.0, y = 0.0 }; }
+    zero :: () -> Vec2 { Vec2.{ x = 0.0, y = 0.0 } }
 
     add :: (self: Vec2, b: Vec2) -> Vec2 {
-        Vec2.{ x = self.x + b.x, y = self.y + b.y };
+        Vec2.{ x = self.x + b.x, y = self.y + b.y }
     }
 
     sub :: (self: Vec2, b: Vec2) -> Vec2 {
-        Vec2.{ x = self.x - b.x, y = self.y - b.y };
+        Vec2.{ x = self.x - b.x, y = self.y - b.y }
     }
 
     scale :: (self: Vec2, s: f32) -> Vec2 {
-        Vec2.{ x = self.x * s, y = self.y * s };
+        Vec2.{ x = self.x * s, y = self.y * s }
     }
 
     dot :: (self: Vec2, b: Vec2) -> f32 {
-        self.x * b.x + self.y * b.y;
+        self.x * b.x + self.y * b.y
     }
 
     length :: (self: Vec2) -> f32 {
-        sqrt(self.x * self.x + self.y * self.y);
+        sqrt(self.x * self.x + self.y * self.y)
     }
 
     normalize :: (self: Vec2) -> Vec2 {
@@ -28,22 +28,22 @@ Vec2 :: struct {
         if len > 0.0 {
             return Vec2.{ x = self.x / len, y = self.y / len };
         }
-        Vec2.zero();
+        Vec2.zero()
     }
 
     lerp :: (self: Vec2, b: Vec2, t: f32) -> Vec2 {
-        Vec2.{ x = self.x + (b.x - self.x) * t, y = self.y + (b.y - self.y) * t };
+        Vec2.{ x = self.x + (b.x - self.x) * t, y = self.y + (b.y - self.y) * t }
     }
 
     distance :: (self: Vec2, b: Vec2) -> f32 {
-        self.sub(b).length();
+        self.sub(b).length()
     }
 
     negate :: (self: Vec2) -> Vec2 {
-        Vec2.{ x = 0.0 - self.x, y = 0.0 - self.y };
+        Vec2.{ x = 0.0 - self.x, y = 0.0 - self.y }
     }
 
     equals :: (self: Vec2, b: Vec2) -> bool {
-        self.x == b.x and self.y == b.y;
+        self.x == b.x and self.y == b.y
     }
 }