feat: named and default parameters

Bootstrap compiler:
- ast.nim: add exprCallArgNames to ekCall for named arg tracking
- parser.nim: detect name: value syntax in call args
- sema.nim: add resolveCallArgs helper that injects default values
  for missing params and reorders named args into param order.
  Parser already parsed default param values; sema now uses them.

Selfhost compiler:
- ast.bux: add defaultExpr to Param, argName to ExprList
- parser.bux: parse = defaultExpr in param list, detect name: value
  syntax in call arguments
- sema.bux: add Sema_ResolveCallArgs with same logic as bootstrap

Tests:
- _test_named_params verifies defaults, named args, mixed positional+named,
  and named args with defaults in both compilers.

All verifications pass: build, selfhost-loop, test-examples, test-golden.

src/ast.bux

@@ -100,6 +100,7 @@ const ekStringInterp: int = 26;
 struct ExprList {
     expr: *Expr,
     next: *ExprList,
+    argName: String,
 }
 
 struct Expr {
@@ -212,6 +213,7 @@ struct Param {
     name: String;
     refParamType: *TypeExpr;
     isVariadic: bool;
+    defaultExpr: *Expr;
 }
 
 struct StructField {