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/1214-ffi-06-callback.sx

@@ -25,7 +25,7 @@ g_callback_sum  : s32 = 0;
 double_it :: (x: s32) -> s32 callconv(.c) {
     g_callback_hits += 1;
     g_callback_sum += x;
-    x * 2;
+    x * 2
 }
 
 add_with_ctx :: (ctx: *void, v: s32) -> s32 callconv(.c) {
@@ -33,7 +33,7 @@ add_with_ctx :: (ctx: *void, v: s32) -> s32 callconv(.c) {
     // Pass a sentinel via ctx to prove the pointer arg also survives the
     // round-trip — read it back as an s32 through *s32.
     p : *s32 = xx ctx;
-    p.* + v;
+    p.* + v
 }
 
 main :: () -> s32 {
@@ -55,5 +55,5 @@ main :: () -> s32 {
     print("ctx + value              = {}\n", r2);
     print("hits after ctx callback  = {}\n", g_callback_hits);
 
-    0;
+    0
 }