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/dock.sx

@@ -81,7 +81,7 @@ dock_zone_to_alignment :: (zone: DockZone) -> ?Alignment {
 }
 
 dock_zone_should_fill :: (zone: DockZone) -> bool {
-    zone == .fill;
+    zone == .fill
 }
 
 // =============================================================================
@@ -169,7 +169,7 @@ DockInteraction :: struct {
 
     get_animated_size :: (self: *DockInteraction, index: s64) -> Size {
         if index >= self.child_count { return Size.zero(); }
-        (@self.anim_sizes.items[index]).current;
+        (@self.anim_sizes.items[index]).current
     }
 
     tick_animations :: (self: *DockInteraction, dt: f32) {
@@ -184,7 +184,7 @@ DockInteraction :: struct {
     get_hovered_dock_zone :: (self: *DockInteraction) -> ?DockZone {
         if self.hovered_zone < 0 { return null; }
         // Map ordinal back to DockZone
-        cast(DockZone) self.hovered_zone;
+        cast(DockZone) self.hovered_zone
     }
 
     set_hovered_dock_zone :: (self: *DockInteraction, zone: ?DockZone) {
@@ -242,7 +242,7 @@ find_hovered_zone :: (bounds: Frame, pos: Point, hint_size: f32, enable_corners:
         if expanded.contains(pos) { return zone; }
         i += 1;
     }
-    null;
+    null
 }
 
 calculate_origin :: (bounds: Frame, child_size: Size, alignment: Alignment) -> Point {
@@ -262,7 +262,7 @@ calculate_origin :: (bounds: Frame, child_size: Size, alignment: Alignment) -> P
         y = bounds.origin.y + bounds.size.height - child_size.height;
     }
 
-    Point.{ x = x, y = y };
+    Point.{ x = x, y = y }
 }
 
 get_size_proposal_for_alignment :: (alignment: Alignment, bounds_size: Size, is_fill: bool) -> ProposedSize {
@@ -280,7 +280,7 @@ get_size_proposal_for_alignment :: (alignment: Alignment, bounds_size: Size, is_
         }
     }
     // Center or corners: natural size
-    ProposedSize.flexible();
+    ProposedSize.flexible()
 }
 
 get_final_size_for_alignment :: (alignment: Alignment, child_size: Size, bounds_size: Size, is_fill: bool) -> Size {
@@ -299,7 +299,7 @@ get_final_size_for_alignment :: (alignment: Alignment, child_size: Size, bounds_
         }
     }
     // Center or corners: natural size
-    child_size;
+    child_size
 }
 
 draw_zone_indicator :: (ctx: *RenderContext, frame: Frame, zone: DockZone, color: Color) {
@@ -385,15 +385,15 @@ DockPanel :: struct {
             header_height = DockPanel.DEFAULT_HEADER_H,
             dock_interaction = null,  // set by Dock.add_panel
             panel_index = 0
-        };
+        }
     }
 }
 
 impl View for DockPanel {
     size_that_fits :: (self: *DockPanel, proposal: ProposedSize) -> Size {
         content_size := self.child.view.size_that_fits(ProposedSize.{ width = proposal.width, height = null });
-        w := if pw := proposal.width { min(content_size.width, pw); } else { content_size.width; };
-        Size.{ width = w, height = content_size.height + self.header_height };
+        w := if pw := proposal.width { min(content_size.width, pw) } else { content_size.width };
+        Size.{ width = w, height = content_size.height + self.header_height }
     }
 
     layout :: (self: *DockPanel, bounds: Frame) {
@@ -446,7 +446,7 @@ impl View for DockPanel {
         }
 
         // Forward to child content
-        self.child.view.handle_event(event, self.child.computed_frame);
+        self.child.view.handle_event(event, self.child.computed_frame)
     }
 }
 
@@ -484,7 +484,7 @@ Dock :: struct {
         d.preview_color = Color.rgba(77, 153, 255, 64);
         d.enable_corners = true;
         d.on_dock = null;
-        d;
+        d
     }
 
     add_panel :: (self: *Dock, panel: DockPanel) {
@@ -508,7 +508,7 @@ impl View for Dock {
         Size.{
             width = proposal.width ?? 800.0,
             height = proposal.height ?? 600.0
-        };
+        }
     }
 
     layout :: (self: *Dock, bounds: Frame) {
@@ -687,6 +687,6 @@ impl View for Dock {
             }
             i -= 1;
         }
-        false;
+        false
     }
 }