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/0415-protocols-protocols.sx

@@ -49,24 +49,24 @@ WindowFlags :: enum flags u32 { vsync :: 64; resizable :: 4; hidden :: 128; }
 
 // --- Top-level functions ---
 
-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 :: () {
@@ -74,19 +74,19 @@ void_return :: () {
 }
 
 implicit_return :: (x: s32) -> s32 {
-    x * 2;
+    x * 2
 }
 
 early_return :: (x: s32) -> s32 {
     if x > 10 { return 99; }
-    x;
+    x
 }
 
 vec3 :: (x: f32, y: f32, z: f32) -> Vector(3, f32) {
-    .[x, y, z];
+    .[x, y, z]
 }
 
-point_sum :: (p: Point) -> s32 { p.x + p.y; }
+point_sum :: (p: Point) -> s32 { p.x + p.y }
 
 // #run compile-time constants
 
@@ -215,11 +215,11 @@ SimpleCounter :: struct { val: s32; }
 
 impl Counter for SimpleCounter {
     inc :: (self: *SimpleCounter) { self.val += 1; }
-    get :: (self: *SimpleCounter) -> s32 { self.val; }
+    get :: (self: *SimpleCounter) -> s32 { self.val }
 }
 
 impl Summable for Point {
-    sum :: (self: *Point) -> s32 { self.x + self.y; }
+    sum :: (self: *Point) -> s32 { self.x + self.y }
 }
 
 // Phase 2: #inline protocol for dynamic dispatch
@@ -236,14 +236,14 @@ Accumulator :: struct {
 
 impl Adder for Accumulator {
     add :: (self: *Accumulator, n: s32) { self.total += n; }
-    value :: (self: *Accumulator) -> s32 { self.total; }
+    value :: (self: *Accumulator) -> s32 { self.total }
 }
 
 Doubler :: struct { val: s32; }
 
 impl Adder for Doubler {
     add :: (self: *Doubler, n: s32) { self.val = self.val + n + n; }
-    value :: (self: *Doubler) -> s32 { self.val; }
+    value :: (self: *Doubler) -> s32 { self.val }
 }
 
 // Phase 4: default methods
@@ -272,10 +272,10 @@ impl Repeater for Printer {
 Chained :: protocol {
     base :: (msg: string) -> s32;
     wrap :: (msg: string) -> s32 {
-        self.base(msg) + 1;
+        self.base(msg) + 1
     }
     double_wrap :: (msg: string) -> s32 {
-        self.wrap(msg) + self.wrap(msg);
+        self.wrap(msg) + self.wrap(msg)
     }
 }
 
@@ -283,7 +283,7 @@ ChainImpl :: struct { val: s32; }
 impl Chained for ChainImpl {
     base :: (self: *ChainImpl, msg: string) -> s32 {
         self.val += 1;
-        msg.len;
+        msg.len
     }
 }
 
@@ -296,7 +296,7 @@ Eq :: protocol {
 
 impl Eq for Point {
     eq :: (self: *Point, other: Point) -> bool {
-        self.x == other.x and self.y == other.y;
+        self.x == other.x and self.y == other.y
     }
 }
 
@@ -306,13 +306,13 @@ Cloneable :: protocol {
 
 impl Cloneable for Point {
     clone :: (self: *Point) -> Point {
-        Point.{ x = self.x, y = self.y };
+        Point.{ x = self.x, y = self.y }
     }
 }
 
 impl Eq for s64 {
     eq :: (self: *s64, other: s64) -> bool {
-        self.* == other;
+        self.* == other
     }
 }
 
@@ -320,7 +320,7 @@ impl Eq for s64 {
 
 // Phase 6: Generic constraints
 are_equal :: ($T: Type/Eq, a: T, b: T) -> bool {
-    a.eq(b);
+    a.eq(b)
 }
 
 Hashable :: protocol {
@@ -329,20 +329,20 @@ Hashable :: protocol {
 
 impl Hashable for Point {
     hash :: (self: *Point) -> s64 {
-        xx self.x * 31 + xx self.y;
+        xx self.x * 31 + xx self.y
     }
 }
 
 eq_and_hash :: ($T: Type/Eq/Hashable, a: T, b: T) -> bool {
     if a.hash() != b.hash() { return false; }
-    a.eq(b);
+    a.eq(b)
 }
 
 // P6.4: inline constraint syntax ($T/Protocol)
 
 // P6.4: inline constraint syntax ($T/Protocol)
 sum_of_inline :: (a: $T/Summable, b: T) -> s32 {
-    a.sum() + b.sum();
+    a.sum() + b.sum()
 }
 
 // Phase 7: Generic struct impls
@@ -355,7 +355,7 @@ Pair :: struct ($T: Type) {
 
 impl Summable for Pair($T) {
     sum :: (self: *Pair(T)) -> s32 {
-        xx self.a + xx self.b;
+        xx self.a + xx self.b
     }
 }
 
@@ -435,7 +435,7 @@ main :: () {
     {
         use_adder :: (a: Adder, n: s32) -> s32 {
             a.add(n);
-            a.value();
+            a.value()
         }
         acc := Accumulator.{ total = 100 };
         result := use_adder(xx @acc, 50);
@@ -470,7 +470,7 @@ main :: () {
         use_counter :: (c: Counter) -> s32 {
             c.inc();
             c.inc();
-            c.get();
+            c.get()
         }
         sc := SimpleCounter.{ val = 10 };
         result := use_counter(xx @sc);
@@ -603,14 +603,14 @@ main :: () {
 
     // P2.7: xx on inline struct literal (no intermediate variable)
     {
-        use_adder :: (a: Adder) -> s32 { a.add(10); a.value(); }
+        use_adder :: (a: Adder) -> s32 { a.add(10); a.value() }
         result := use_adder(xx Accumulator.{ total = 5 });
         print("P2.7: {}\n", result);
     }
 
     // P3.3: xx on inline struct literal with vtable protocol
     {
-        use_counter :: (c: Counter) -> s32 { c.inc(); c.inc(); c.get(); }
+        use_counter :: (c: Counter) -> s32 { c.inc(); c.inc(); c.get() }
         result := use_counter(xx SimpleCounter.{ val = 100 });
         print("P3.3: {}\n", result);
     }