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/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
     }
 }