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.

bootstrap/ast.nim

@@ -174,6 +174,7 @@ type
     of ekCall:
       exprCallCallee*: Expr
       exprCallArgs*: seq[Expr]
+      exprCallArgNames*: seq[string]  ## empty = positional, non-empty = named arg
       exprCallInferredTypeArgs*: seq[TypeExpr]  ## filled by sema for inferred generic calls
     of ekGenericCall:
       exprGenericCallee*: string