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.

docs/ROADMAP.md

@@ -81,21 +81,21 @@ let msg2: String = "Count: {num}";
 
 ---
 
-### 5. Named / Default Parameters
+### 5. Named / Default Parameters ✅ DONE
 **Why:** API ergonomics.
 
 **Syntax:**
 ```bux
-func HttpResponse(code: int = 200, contentType: String = "text/plain", body: String = "") -> Response { ... }
-let r: Response = HttpResponse(body: "hello");  // code=200, contentType=default
+func HttpResponse(code: int = 200, body: String = "") -> Response { ... }
+let r: Response = HttpResponse(body: "hello");  // code=200
+let s = HttpResponse(404, body: "err");         // positional + named mixed
 ```
 
-**Implementation Steps:**
-1. Parser: allow `param: Type = defaultExpr`
-2. Sema: fill missing args at call sites
-3. C backend: emit args in correct order with defaults
-
-**Complexity:** Medium — sema changes for call resolution.
+**Status:** Implemented in both bootstrap and selfhost.
+- Bootstrap parser already parsed defaults; added named-arg parsing and sema injection.
+- Selfhost parser now parses `= defaultExpr` in params and `name: value` at call sites.
+- Sema injects default expressions and reorders named args into param order.
+- HIR lowerer unchanged — desugaring happens in sema.
 
 ---