datalog: parser accepts negative integer literals (248/248)

Bug: `n(-1).` failed to parse — the tokenizer produced op `-`
followed by number `1`, and dl-pp-parse-arg expected a term after
seeing `-` as an op (and a `(` for a compound) but found a bare
number. Users had to write `(- 0 1)` or compute via `is`.

Fix: dl-pp-parse-arg detects op `-` directly followed by a number
token (no intervening `(`) and consumes both as a single negative
number literal. Subtraction (`is(Y, -(X, 2))`) and compound
arithmetic via the operator form are unaffected — they use the
`-(` lookahead path.

2 new parser tests: negative integer literal and subtraction
compound preserved.

lib/datalog/scoreboard.json

@@ -1,11 +1,11 @@
 {
   "lang": "datalog",
-  "total_passed": 246,
+  "total_passed": 248,
   "total_failed": 0,
-  "total": 246,
+  "total": 248,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
-    {"name":"parse","passed":20,"failed":0,"total":20},
+    {"name":"parse","passed":22,"failed":0,"total":22},
     {"name":"unify","passed":28,"failed":0,"total":28},
     {"name":"eval","passed":36,"failed":0,"total":36},
     {"name":"builtins","passed":23,"failed":0,"total":23},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":34,"failed":0,"total":34},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-10T20:51:33+00:00"
+  "generated": "2026-05-10T20:55:23+00:00"
 }