AST: grow arena/pool for session re-emit (compact z data, negligible image cost)

MaxNodes 65536->262144, MaxTxt 16384->65536, PoolSize 1->4 MiB.  The
arena/pool data lines are compact z runs, so the image text barely
changes; this lifts the capacity limit that overflowed when re-emitting
the whole compiler session.  Suite 204/204; FIXPOINT OK (3,405,864).

compiler/src/AST.mod

@@ -12,9 +12,9 @@ IMPLEMENTATION MODULE AST;
 IMPORT FileIO;
 
 CONST
-  MaxNodes = 65536;
-  MaxTxt   = 16384;
-  PoolSize = 1048576;   (* 1 MiB of pooled text *)
+  MaxNodes = 262144;
+  MaxTxt   = 65536;
+  PoolSize = 4194304;   (* 4 MiB of pooled text *)
   MaxTxtLen = 255;
 
   (* arena has MaxNodes slots, indices 0..MaxNodes-1 *)