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).

library/modules/ui/animation.sx

@@ -9,14 +9,14 @@ Lerpable :: protocol #inline {
 
 // --- Easing Functions ---
 
-ease_linear :: (t: f32) -> f32 { t; }
-ease_in_quad :: (t: f32) -> f32 { t * t; }
-ease_out_quad :: (t: f32) -> f32 { t * (2.0 - t); }
+ease_linear :: (t: f32) -> f32 { t }
+ease_in_quad :: (t: f32) -> f32 { t * t }
+ease_out_quad :: (t: f32) -> f32 { t * (2.0 - t) }
 ease_in_out_quad :: (t: f32) -> f32 {
     if t < 0.5 then 2.0 * t * t
-    else -1.0 + (4.0 - 2.0 * t) * t;
+    else -1.0 + (4.0 - 2.0 * t) * t
 }
-ease_out_cubic :: (t: f32) -> f32 { u := t - 1.0; u * u * u + 1.0; }
+ease_out_cubic :: (t: f32) -> f32 { u := t - 1.0; u * u * u + 1.0 }
 
 // --- AnimatedFloat — duration-based ---
 
@@ -38,7 +38,7 @@ AnimatedFloat :: struct {
             duration = 0.0,
             easing = null,
             active = false
-        };
+        }
     }
 
     animate_to :: (self: *AnimatedFloat, target: f32, dur: f32, ease: Closure(f32) -> f32) {
@@ -54,7 +54,7 @@ AnimatedFloat :: struct {
         if !self.active { return; }
         self.elapsed += dt;
         t := clamp(self.elapsed / self.duration, 0.0, 1.0);
-        eased := if ease := self.easing { ease(t); } else { t; };
+        eased := if ease := self.easing { ease(t) } else { t };
         self.current = self.from + (self.to - self.from) * eased;
         if t >= 1.0 {
             self.current = self.to;
@@ -83,7 +83,7 @@ SpringFloat :: struct {
             damping = 20.0,
             mass = 1.0,
             threshold = 0.01
-        };
+        }
     }
 
     snappy :: (value: f32) -> SpringFloat {
@@ -95,7 +95,7 @@ SpringFloat :: struct {
             damping = 25.0,
             mass = 1.0,
             threshold = 0.01
-        };
+        }
     }
 
     tick :: (self: *SpringFloat, dt: f32) {
@@ -109,7 +109,7 @@ SpringFloat :: struct {
 
     is_settled :: (self: *SpringFloat) -> bool {
         abs(self.current - self.target) < self.threshold
-        and abs(self.velocity) < self.threshold;
+        and abs(self.velocity) < self.threshold
     }
 }
 
@@ -131,7 +131,7 @@ Animated :: struct ($T: Lerpable) {
             elapsed = 0.0,
             duration = 0.0,
             active = false
-        };
+        }
     }
 
     // Jump immediately to value (no animation). Used to avoid animating from zero on first layout.
@@ -163,5 +163,5 @@ Animated :: struct ($T: Lerpable) {
         }
     }
 
-    is_animating :: (self: *Animated(T)) -> bool { self.active; }
+    is_animating :: (self: *Animated(T)) -> bool { self.active }
 }