fix(migrate): 0.R4 - --prune keeps a state declared after a plain parameter; a parameter named twice is refused

On Maroon Lake the prune gave home_keep_records(base_app_st, home_st, r: RunRecords, save_app_st) a
second save_app_st at the front, and every call a second argument: a state declared after a plain
parameter was not counted as declared. It is now, and a call to such a function is never given the
state again. `ludic build --check` let the duplicate through and clang refused it; the checker now
refuses a function that names two parameters alike (`add names two parameters n`).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

selfhost/check/check_stmt.ludic

@@ -154,12 +154,26 @@ function ck_stmt(s: Node) -> void {
   }
   ck_walk(s)
 }
+# a name given to two parameters: the second would shadow the first, and nothing could reach it
+function ck_param_twice(d: Node, i: int) -> void {
+  var j = 0
+  while j < i {
+    if d.kids[j].kind == N_PARAM and (d.kids[j].s == d.kids[i].s) {
+      ck_err("arity", d.kids[i], `{d.s} names two parameters {d.kids[i].s}`)
+      return
+    }
+    j += 1
+  }
+}
 function ck_fn_body(d: Node) -> void {
   mg_enter(d, 0)
   let m = ck_mark()
   var i = 0
   while i < len(d.kids) {
-    if d.kids[i].kind == N_PARAM { ck_bind_param(d.kids[i]) }
+    if d.kids[i].kind == N_PARAM {
+      ck_param_twice(d, i)
+      ck_bind_param(d.kids[i])
+    }
     i += 1
   }
   ck_ret = d.ty

selfhost/frontend/migrate.ludic

@@ -559,8 +559,22 @@ function mg_list(set: pointer) -> []int {
 function mg_declared(n: Node, s: int) -> bool {
   if n.kind != N_FN { return false }
   var i = 0
-  while i < len(n.kids) and n.kids[i].kind == N_PARAM and is_state_ty(n.kids[i].ty) {
-    if (n.kids[i].ty == g_ms_name[s]) { return true }
+  while i < len(n.kids) {                        # anywhere in the list: a state after a plain parameter is declared too
+    if n.kids[i].kind == N_PARAM and (n.kids[i].ty == g_ms_name[s]) { return true }
+    i += 1
+  }
+  return false
+}
+# a state declared after a plain parameter: every call already names it where it stands
+function mg_declared_late(n: Node, s: int) -> bool {
+  if n.kind != N_FN { return false }
+  var plain = false
+  var i = 0
+  while i < len(n.kids) {
+    if n.kids[i].kind == N_PARAM {
+      if not is_state_ty(n.kids[i].ty) { plain = true }
+      else if plain and (n.kids[i].ty == g_ms_name[s]) { return true }
+    }
     i += 1
   }
   return false
@@ -972,7 +986,7 @@ function mg_calls() -> void {
       let passed = g_mc_passed[c] == 1
       var k = 0
       while k < len(need) {
-        if not (mg_declared(callee, need[k]) and passed) {
+        if not (mg_declared(callee, need[k]) and (passed or mg_declared_late(callee, need[k]))) {
           if len(text) > 0 { text = text + ", " }
           text = text + g_ms_param[need[k]]
         }