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/glyph_cache.sx

@@ -28,16 +28,16 @@ GlyphEntry :: struct {
 // Quantize font size to half-point increments to limit cache entries.
 // e.g., 13.0 -> 26, 13.5 -> 27, 14.0 -> 28
 quantize_size :: (font_size: f32) -> u16 {
-    xx (font_size * 2.0 + 0.5);
+    xx (font_size * 2.0 + 0.5)
 }
 
 dequantize_size :: (q: u16) -> f32 {
-    xx q / 2.0;
+    xx q / 2.0
 }
 
 // Pack (glyph_index, size_quantized) into a single u32 for fast comparison
 make_glyph_key :: (glyph_index: u16, size_quantized: u16) -> u32 {
-    (xx glyph_index << 16) | xx size_quantized;
+    (xx glyph_index << 16) | xx size_quantized
 }
 
 // Shaped glyph — output of text shaping (positioned glyph with index)
@@ -54,7 +54,7 @@ is_ascii :: (text: string) -> bool {
         if text[i] >= 128 { return false; }
         i += 1;
     }
-    true;
+    true
 }
 
 // kbts constants (C enum values)
@@ -480,7 +480,7 @@ GlyphCache :: struct {
         }
 
         // No space
-        PackResult.{ x = 0 - 1, y = 0 - 1 };
+        PackResult.{ x = 0 - 1, y = 0 - 1 }
     }
 
     // Grow the atlas by doubling dimensions
@@ -547,18 +547,18 @@ GlyphCache :: struct {
 
     // Get the scale factor for a logical font size
     scale_for_size :: (self: *GlyphCache, font_size: f32) -> f32 {
-        stbtt_ScaleForPixelHeight(self.font_info, font_size);
+        stbtt_ScaleForPixelHeight(self.font_info, font_size)
     }
 
     // Get scaled ascent for a logical font size
     get_ascent :: (self: *GlyphCache, font_size: f32) -> f32 {
-        self.ascent * self.scale_for_size(font_size);
+        self.ascent * self.scale_for_size(font_size)
     }
 
     // Get scaled line height for a logical font size
     get_line_height :: (self: *GlyphCache, font_size: f32) -> f32 {
         s := self.scale_for_size(font_size);
-        (self.ascent - self.descent + self.line_gap) * s;
+        (self.ascent - self.descent + self.line_gap) * s
     }
 
     // Shape text into positioned glyphs.
@@ -654,6 +654,6 @@ GlyphCache :: struct {
             width += self.shaped_buf.items[i].advance;
             i += 1;
         }
-        Size.{ width = width, height = self.get_line_height(font_size) };
+        Size.{ width = width, height = self.get_line_height(font_size) }
     }
 }