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/parser.sx

@@ -91,6 +91,16 @@
           ((= ty "number") (do (dl-pp-advance! st) vv))
           ((= ty "string") (do (dl-pp-advance! st) vv))
           ((= ty "var") (do (dl-pp-advance! st) (string->symbol vv)))
+          ;; Negative numeric literal: `-` op directly followed by a
+          ;; number (no `(`) is parsed as a single negative number.
+          ;; This keeps `(-X Y)` (compound) and `-N` (literal) distinct.
+          ((and (= ty "op") (= vv "-")
+                (= (get (dl-pp-peek2 st) :type) "number"))
+            (do
+              (dl-pp-advance! st)
+              (let
+                ((n (get (dl-pp-peek st) :value)))
+                (do (dl-pp-advance! st) (- 0 n)))))
           ((or (= ty "atom") (= ty "op"))
             (do
               (dl-pp-advance! st)