Adds a user-facing strategy hook: dl-set-strategy! db strategy and
dl-get-strategy db. Default :semi-naive; :magic is accepted but
the actual transformation is deferred — the saturator currently
falls back to semi-naive regardless. Lets us tick the Phase 6
"Optional pass — guarded behind dl-set-strategy!" checkbox while
keeping the equivalence/perf tests pending future work.
3 new eval tests.
Returns {<rel-name>: tuple-count} for relations with any tuples
or that are rule-headed (so empty IDB shows as :rel 0 rather than
disappearing). Skips placeholder entries from internal
dl-ensure-rel! calls. 4 tests cover basic, empty IDB, mixed
EDB+IDB, and empty-db cases.
(p X _), (p _ Y) — the two _ are now different variables, matching
standard Datalog semantics. Previously both _ symbols were the same
SX symbol, so unification across them gave wrong answers.
Fix in db.sx: dl-rename-anon-term + dl-rename-anon-lit walk a term
or literal and replace each '_' symbol with a fresh _anon<N>.
dl-make-anon-renamer returns a counter-based name generator scoped
per call. dl-rename-anon-rule applies it to head and body of a
rule. dl-add-rule! invokes the renamer before safety check.
eval.sx: dl-query renames anon vars in the goal before search and
filters '_' out of the projection so user-facing results aren't
polluted with internal _anon<N> bindings.
The previous "underscore in head ok" test now correctly rejects
(p X _) :- q(X) as unsafe (the head's fresh anon var has no body
binder). New "underscore in body only" test confirms the safe
case. Two regression tests for rule-level and goal-level
independence.
