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/1335-ffi-objc-call-09-in-construct.sx

@@ -18,7 +18,7 @@
 Probe :: struct {
     receiver: *void = null;
     fetch :: (self: *Probe) -> s64 {
-        #objc_call(s64)(self.receiver, "hash");
+        #objc_call(s64)(self.receiver, "hash")
     }
 }
 
@@ -29,7 +29,7 @@ Hashable :: protocol {
 
 impl Hashable for Probe {
     sx_hash :: (self: *Probe) -> s64 {
-        #objc_call(s64)(self.receiver, "hash") * 2;
+        #objc_call(s64)(self.receiver, "hash") * 2
     }
 }
 
@@ -38,13 +38,13 @@ impl Hashable for Probe {
 // references it inside the `#objc_call` arg list. Locked in by
 // `examples/103-ffi-closure-capture.sx`.
 make_hasher :: (recv: *void) -> Closure(s32) -> s64 {
-    closure((dummy: s32) -> s64 => #objc_call(s64)(recv, "hash"));
+    closure((dummy: s32) -> s64 => #objc_call(s64)(recv, "hash"))
 }
 
 // ── 4. Generic function body — instantiated per call site ───────────
 hash_through :: (recv: $T) -> s64 {
     p : *void = xx recv;
-    #objc_call(s64)(p, "hash");
+    #objc_call(s64)(p, "hash")
 }
 
 main :: () -> s32 {
@@ -76,5 +76,5 @@ main :: () -> s32 {
     inline if OS != .macos {
         print("skipped (not macos)\n");
     }
-    0;
+    0
 }