feat: multi-stmt match arms and block-as-expression

Blocks can yield a value: last expression statement is the result.
Match arms accept block bodies, so multi-statement arms work:

  match n {
    1 => { let a = 10; a + 1 },
    _ => 0
  }

Bootstrap: lowerBlock(asExpr) lifts the trailing skExpr. Selfhost: parse
{...} as ekBlock, emit __blk_N yield temps (retTypeKind -2), and treat
only non-empty strValue as match/block yield. Nested enum/struct pattern
bindings recurse in both compilers.

Example: match_block.bux. Selfhost-loop remains binary-identical.

docs/LanguageRef.md

@@ -396,6 +396,21 @@ match p {
     Point { x, y } => x * 10 + y,
     _ => -1
 }
+
+// Multi-statement arm bodies (block expression; last expr is the value)
+match n {
+    1 => {
+        let a: int = 10;
+        a + 1
+    },
+    _ => 0
+}
+
+// Block as expression
+let r: int = {
+    let x: int = 5;
+    x + 6
+};
 ```
 
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