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/0028-basic-functions.sx

@@ -4,24 +4,24 @@
 #import "modules/test.sx";
 pkg :: #import "modules/testpkg";
 
-add :: (a: s32, b: s32) -> s32 { a + b; }
+add :: (a: s32, b: s32) -> s32 { a + b }
 
-mul :: (a: s32, b: s32) -> s32 { a * b; }
+mul :: (a: s32, b: s32) -> s32 { a * b }
 
-identity :: (x: $T) -> T { x; }
+identity :: (x: $T) -> T { x }
 
 pair_add :: (a: $T, b: $U) -> s64 {
-    cast(s64) a + cast(s64) b;
+    cast(s64) a + cast(s64) b
 }
 
 typed_sum :: (..args: []s32) -> s32 {
     result := 0;
     for args: (it) { result = result + it; }
-    result;
+    result
 }
 
 apply :: (f: (s32, s32) -> s32, x: s32, y: s32) -> s32 {
-    f(x, y);
+    f(x, y)
 }
 
 void_return :: () {
@@ -29,12 +29,12 @@ void_return :: () {
 }
 
 implicit_return :: (x: s32) -> s32 {
-    x * 2;
+    x * 2
 }
 
 early_return :: (x: s32) -> s32 {
     if x > 10 { return 99; }
-    x;
+    x
 }
 
 main :: () {
@@ -84,7 +84,7 @@ main :: () {
     print("lambda-ret: {}\n", halve(10.0));
 
     // Local function (non-lambda)
-    local_add :: (a: s32, b: s32) -> s32 { a + b; }
+    local_add :: (a: s32, b: s32) -> s32 { a + b }
     print("local-fn: {}\n", local_add(3, 4));
 
     // Nested function calls