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.md

@@ -1,11 +1,11 @@
 # datalog scoreboard
 
-**246 / 246 passing** (0 failure(s)).
+**248 / 248 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 26 | 26 | ok |
-| parse | 20 | 20 | ok |
+| parse | 22 | 22 | ok |
 | unify | 28 | 28 | ok |
 | eval | 36 | 36 | ok |
 | builtins | 23 | 23 | ok |