escape (fix): a node walked more than once is scratch only if every walk found it LOCAL

A field's default is one expression, walked at every 'new' of its record: marked LOCAL by a frame's
temporary, it stayed marked when a record a pool keeps was made from it, and that record's list came
from the frame's scratch. The marks are now taken off any node one walk found kept.
examples/lang/arena_defaults.ludic is the case (a pool's record made in frame 3, a temporary of the
same type every frame): foundations 1315baf crashes on it poisoned; this prints '497 124747', as the
heap does. In ludic-dev test.

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

selfhost/backend/emit_escape.ludic

@@ -627,6 +627,9 @@ function escape_analyse() -> void {
     li += 1
   }
   es_solve()
+  # a node analysed more than once (a field's default, walked at every `new` of its record) is
+  # scratch only if every walk found it LOCAL: mark the LOCAL ones, then take the mark off any node
+  # one walk found kept
   var s = 0
   while s < len(g_es_site) {
     let n = g_es_site[s]
@@ -648,6 +651,15 @@ function escape_analyse() -> void {
     }
     s += 1
   }
+  var u = 0
+  while u < len(g_es_site) {
+    let n2 = g_es_site[u]
+    let c2 = g_es_site_cls[u]
+    let fi2 = g_es_site_fn[u]
+    let spans2 = fi2 < 0 or g_es_fspans[fi2]
+    if n2.uns == ES_SCRATCH and (c2 < 0 or (g_es_flag[c2] & (ES_ESC | ES_HEAP)) != 0 or spans2) { n2.uns = 0 }
+    u += 1
+  }
 }
 
 var g_es_report: bool = false