datalog: anonymous-renamer avoids user _anon<N> collision

The renamer for anonymous `_` variables started at counter 0 and
produced `_anon1, _anon2, ...` unconditionally. A user writing the
same naming convention would see their variables shadowed:

  (dl-eval "p(a, b). p(c, d). q(_anon1) :- p(_anon1, _)."
           "?- q(X).")
  => ()    ; should be ({:X a} {:X c})

The `_` got renamed to `_anon1` too, collapsing the two positions
of `p` to a single var (forcing args to be equal — which neither
tuple satisfies).

Fix: scan each rule (and query goal) for the highest `_anon<N>`
already present and start the renamer past it. New helpers
`dl-max-anon-num` / `dl-max-anon-num-list` / `dl-try-parse-int`
walk the rule tree; `dl-make-anon-renamer` now takes a `start`
argument; `dl-rename-anon-rule` and the query-time renamer in
`dl-query` both compute the start from the input.

1 regression test; conformance 275/275.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/datalog/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 274,
+  "total_passed": 275,
   "total_failed": 0,
-  "total": 274,
+  "total": 275,
   "suites": [
     {"name":"tokenize","passed":31,"failed":0,"total":31},
     {"name":"parse","passed":23,"failed":0,"total":23},
     {"name":"unify","passed":29,"failed":0,"total":29},
-    {"name":"eval","passed":42,"failed":0,"total":42},
+    {"name":"eval","passed":43,"failed":0,"total":43},
     {"name":"builtins","passed":26,"failed":0,"total":26},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":12,"failed":0,"total":12},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":37,"failed":0,"total":37},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-11T08:59:09+00:00"
+  "generated": "2026-05-11T09:34:17+00:00"
 }