feat: pattern bindings, empty closures, match-as-expr, string interp

Sessions 10–12 from QUALITY_PLAN:

- Pattern payload bindings (Some(value) => value) in bootstrap and selfhost
- Empty-param closures via || (tkPipePipe) with loop/return bodies
- Expression-form match: let x = match …; newline before arms
- f"…" string interpolation desugared to String_Concat + conversions
- Lexer preserves \{ \} for literal braces in f-strings
- Bootstrap fix: f"plain" strips the f prefix after escape processing
- Examples: pattern_matching, closure_control, match_let, string_interp

Selfhost-loop remains binary-identical; all examples and error goldens pass.

src/ast.bux

@@ -79,6 +79,8 @@ struct Pattern {
     patStructName: String,   // for pkStruct
     patChild1: *Pattern,     // range lo / nested
     patChild2: *Pattern,     // range hi / nested
+    patArgs: *Pattern,       // pkEnum payload args (head)
+    patNext: *Pattern,       // next sibling in patArgs list
 }
 
 // Match arm: pattern => body