feat: match guards, HOF inference, ownership C.2/C.3, LSP sema hover

Sessions 18–23 quality work:

- B.3c match arm guards + sequential found-flag lower (bootstrap + selfhost)
- Generic HOF type inference (Array/Iter map/filter/fold without type args)
- Pattern binding shadowing via unique C locals (__pN_src)
- Ownership C.2 exclusive &mut data-flow + C.4 goldens; *p= store-through fix
- Ownership C.3 auto-drop on early return/branches: scoped defers, move-on-return,
  Drop monomorphization, materialize return before Drop
- LSP 0.3.0: hover from real sema types
- Examples and QUALITY_PLAN session log; selfhost-loop identical

src/ast.bux

@@ -66,6 +66,7 @@ const pkRange: int = 3;
 const pkEnum: int = 4;
 const pkStruct: int = 5;
 const pkTuple: int = 6;
+const pkGuarded: int = 7;    // `p if cond` — patChild1 = inner, patGuardExpr = condition
 
 struct Pattern {
     kind: int,
@@ -78,10 +79,11 @@ struct Pattern {
     patEnumPath: String,     // for pkEnum: "Enum::Variant"
     patStructName: String,   // for pkStruct (type name)
     patFieldName: String,    // for struct field entry: field name in Point { x: a }
-    patChild1: *Pattern,     // range lo / nested
+    patChild1: *Pattern,     // range lo / nested / guarded inner
     patChild2: *Pattern,     // range hi / nested
     patArgs: *Pattern,       // pkEnum/pkTuple/pkStruct field list (head)
     patNext: *Pattern,       // next sibling in patArgs list
+    patGuardExpr: *Expr,     // for pkGuarded: the `if` condition
 }
 
 // Match arm: pattern => body