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/1216-ffi-08-foreign-in-method.sx

@@ -22,7 +22,7 @@ Counter :: struct {
     next :: (self: *Counter) -> s32 {
         v := ffi_method_helper(self.seed);
         self.seed += 1;
-        v;
+        v
     }
 }
 
@@ -33,13 +33,13 @@ Doubler :: protocol {
 
 impl Doubler for Counter {
     doubled :: (self: *Counter) -> s32 {
-        ffi_method_helper(self.seed) * 2;
+        ffi_method_helper(self.seed) * 2
     }
 }
 
 // ── 3. Closure body calling a #foreign fn ────────────────────────────
 make_adder :: (bias: s32) -> Closure(s32) -> s32 {
-    closure((x: s32) -> s32 => ffi_method_helper(x) + bias);
+    closure((x: s32) -> s32 => ffi_method_helper(x) + bias)
 }
 
 main :: () -> s32 {
@@ -66,5 +66,5 @@ main :: () -> s32 {
         else:        { print("inline if other = {}\n", ffi_method_helper(7)); }
     }
 
-    0;
+    0
 }