feat: struct and tuple patterns in match (bootstrap + selfhost)

Support destructuring in match arms:
- Tuple: (a, b) binds subject._0 / _1
- Struct: Point { x: px, y: py } and shorthand Point { x, y }

Bootstrap: matchPatternBindings for pkTuple/pkStruct; register local
tuple typedefs from function bodies. Selfhost: parse, Sema_BindPattern,
Lcx_PatternBindings with scope defines. Fix operator-overload path that
crashed when typeName was null after pattern binds.

Example: examples/struct_tuple_pat.bux. Selfhost-loop IDENTICAL.

docs/LanguageRef.md

@@ -383,7 +383,20 @@ Supported patterns:
 - Identifier catch-all: `name` (binds whole subject)
 - Range: `1..9`, `1..=9`
 - Enum tags + **payload bindings**: `Option::Some(value)`, `Pair::Two(a, b)`
-- Struct / tuple / guard patterns: parsed; full lowering still evolving
+- **Tuple patterns**: `(a, b)` → binds `subject._0`, `subject._1`
+- **Struct patterns**: `Point { x: px, y: py }` or shorthand `Point { x, y }`
+- Guard patterns: parsed; full lowering still evolving
+
+```bux
+match pair {
+    (a, b) => a + b,
+    _ => 0
+}
+match p {
+    Point { x, y } => x * 10 + y,
+    _ => -1
+}
+```
 
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