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/stacks.sx

@@ -18,7 +18,7 @@ VStack :: struct {
 
 impl View for VStack {
     size_that_fits :: (self: *VStack, proposal: ProposedSize) -> Size {
-        measure_vstack(@self.children, proposal, self.spacing);
+        measure_vstack(@self.children, proposal, self.spacing)
     }
 
     layout :: (self: *VStack, bounds: Frame) {
@@ -44,7 +44,7 @@ impl View for VStack {
             }
             i -= 1;
         }
-        false;
+        false
     }
 }
 
@@ -60,7 +60,7 @@ HStack :: struct {
 
 impl View for HStack {
     size_that_fits :: (self: *HStack, proposal: ProposedSize) -> Size {
-        measure_hstack(@self.children, proposal, self.spacing);
+        measure_hstack(@self.children, proposal, self.spacing)
     }
 
     layout :: (self: *HStack, bounds: Frame) {
@@ -85,7 +85,7 @@ impl View for HStack {
             }
             i -= 1;
         }
-        false;
+        false
     }
 }
 
@@ -100,7 +100,7 @@ ZStack :: struct {
 
 impl View for ZStack {
     size_that_fits :: (self: *ZStack, proposal: ProposedSize) -> Size {
-        measure_zstack(@self.children, proposal);
+        measure_zstack(@self.children, proposal)
     }
 
     layout :: (self: *ZStack, bounds: Frame) {
@@ -127,7 +127,7 @@ impl View for ZStack {
             }
             i -= 1;
         }
-        false;
+        false
     }
 }
 
@@ -142,12 +142,12 @@ impl View for Spacer {
     size_that_fits :: (self: *Spacer, proposal: ProposedSize) -> Size {
         w := proposal.width ?? self.min_length;
         h := proposal.height ?? self.min_length;
-        Size.{ width = max(w, self.min_length), height = max(h, self.min_length) };
+        Size.{ width = max(w, self.min_length), height = max(h, self.min_length) }
     }
 
     layout :: (self: *Spacer, bounds: Frame) {}
     render :: (self: *Spacer, ctx: *RenderContext, frame: Frame) {}
-    handle_event :: (self: *Spacer, event: *Event, frame: Frame) -> bool { false; }
+    handle_event :: (self: *Spacer, event: *Event, frame: Frame) -> bool { false }
 }
 
 // Rect — simple colored rectangle view
@@ -164,7 +164,7 @@ impl View for RectView {
     size_that_fits :: (self: *RectView, proposal: ProposedSize) -> Size {
         w := proposal.width ?? self.preferred_width;
         h := proposal.height ?? self.preferred_height;
-        Size.{ width = w, height = h };
+        Size.{ width = w, height = h }
     }
 
     layout :: (self: *RectView, bounds: Frame) {}
@@ -177,5 +177,5 @@ impl View for RectView {
         }
     }
 
-    handle_event :: (self: *RectView, event: *Event, frame: Frame) -> bool { false; }
+    handle_event :: (self: *RectView, event: *Event, frame: Frame) -> bool { false }
 }