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
 }