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/0036-basic-ufcs-aliases.sx

@@ -10,7 +10,7 @@ main :: () {
     {
         print("=== UFCS Aliases ===\n");
 
-        num_sum :: (a: s64, b: s64) -> s64 { a + b; }
+        num_sum :: (a: s64, b: s64) -> s64 { a + b }
         sum :: ufcs num_sum;
 
         print("{}\n", num_sum(40, 2));   // 42 — direct call
@@ -20,7 +20,7 @@ main :: () {
         print("{}\n", num_sum(40, 2));   // 42 — direct (was tuple full-splat)
         print("{}\n", 40 |> sum(2));     // 42 — pipe (was tuple partial-splat)
 
-        compute :: (a: s64, b: s64, c: s64, d: s64) -> s64 { a + b * c - d; }
+        compute :: (a: s64, b: s64, c: s64, d: s64) -> s64 { a + b * c - d }
         calc :: ufcs compute;
 
         print("{}\n", compute(1, 2, 3, 4));    // 1+2*3-4 = 3 (was tuple full-splat)
@@ -28,14 +28,14 @@ main :: () {
         print("{}\n", 1 |> calc(2, 3, 4));     // same = 3 — pipe UFCS
 
         // Tuple return type
-        swap :: (a: s64, b: s64) -> (s64, s64) { (b, a); }
+        swap :: (a: s64, b: s64) -> (s64, s64) { (b, a) }
         s := swap(1, 2);
         a := s.0;
         b := s.1;
         print("{}\n", a);                // 2
         print("{}\n", b);                // 1
 
-        wrap :: (x: s64) -> (s64) { (x,); }
+        wrap :: (x: s64) -> (s64) { (x,) }
         t := wrap(99);
         print("{}\n", t.0);             // 99
     }