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/0404-protocols-dot-shorthand-protocol-field.sx

@@ -13,18 +13,18 @@ Drawable :: protocol {
 
 Circle :: struct { radius: s32; }
 impl Drawable for Circle {
-    draw :: (self: *Circle) -> s32 { self.radius; }
-    name :: (self: *Circle) -> string { "circle"; }
-    layout :: (self: *Circle, x: s32) -> s32 { x + self.radius; }
-    handle :: (self: *Circle, event: s32) -> bool { event > 0; }
+    draw :: (self: *Circle) -> s32 { self.radius }
+    name :: (self: *Circle) -> string { "circle" }
+    layout :: (self: *Circle, x: s32) -> s32 { x + self.radius }
+    handle :: (self: *Circle, event: s32) -> bool { event > 0 }
 }
 
 Square :: struct { side: s32; }
 impl Drawable for Square {
-    draw :: (self: *Square) -> s32 { self.side * self.side; }
-    name :: (self: *Square) -> string { "square"; }
-    layout :: (self: *Square, x: s32) -> s32 { x + self.side; }
-    handle :: (self: *Square, event: s32) -> bool { event > 1; }
+    draw :: (self: *Square) -> s32 { self.side * self.side }
+    name :: (self: *Square) -> string { "square" }
+    layout :: (self: *Square, x: s32) -> s32 { x + self.side }
+    handle :: (self: *Square, event: s32) -> bool { event > 1 }
 }
 
 Rect :: struct {
@@ -32,7 +32,7 @@ Rect :: struct {
     y: f32;
     w: f32;
     h: f32;
-    zero :: () -> Rect { Rect.{ x = 0.0, y = 0.0, w = 0.0, h = 0.0 }; }
+    zero :: () -> Rect { Rect.{ x = 0.0, y = 0.0, w = 0.0, h = 0.0 } }
 }
 
 Container :: struct {