datalog: first-arg index per relation (Phase 5e perf, 169/169)

db gains :facts-index {<rel>: {<first-arg-key>: tuples}} mirroring
the membership :facts-keys index. dl-add-fact! populates the index;
dl-match-positive walks the body literal's first arg under the
current subst — when it's bound to a non-var, look up by (str arg)
instead of scanning the full relation.

For chain-style recursive rules (parent X Y), (ancestor Y Z) the
inner Y has at most one parent, so the inner lookup returns 0–1
tuples instead of N. chain-25 saturation drops from ~33s to ~18s
real (~2x). chain-50 still long but tractable; next bottleneck is
subst dict copies during unification.

dl-retract! refreshed to keep the new index consistent: kept-index
rebuilt during EDB filter, IDB wipes clear all three slots.

Differential semi-naive test bumped to chain-12, semi-only count
test to chain-25.

plans/datalog-on-sx.md

@@ -264,19 +264,33 @@ large graphs.
 
 ## Blockers
 
-- **Saturation perf** improving but not free. Resolved hash-set
-  membership in `dl-add-fact!` and replaced recursive `(rest lits)` in
-  `dl-find-bindings` with indexed iteration. chain-15 drops from ~25s
-  to ~16s and chain-25 saturates in ~33s real / 11s user — most CPU
-  now in unification (assoc-based subst dict copies) and dict
-  lookups during walks. Future: a per-rule "compiled" body that
-  pre-resolves arg positions and intern variable indices, then
-  unification can use array slots instead of dict assoc.
+- **Saturation perf**: three rounds done.
+  - hash-set membership in `dl-add-fact!` (Phase 5b)
+  - indexed iteration in `dl-find-bindings` (Phase 5c)
+  - first-arg index per relation (Phase 5e) — when a body literal's
+    first arg walks to a non-variable, dl-match-positive looks up
+    by `(str arg)` instead of scanning the full relation.
+  chain-25 saturation drops from ~33s to ~18s real (10s user).
+  chain-50 still long (~120s+) due to dict-copy overhead in
+  unification subst threading. Future: per-rule "compiled" body
+  with pre-resolved var positions, slot-based subst representation
+  to avoid `assoc` per binding.
 
 ## Progress log
 
 _Newest first._
 
+- 2026-05-08 — Phase 5e perf: first-arg index per relation. db gains
+  `:facts-index {<rel>: {<first-arg-key>: tuples}}` mirroring the
+  existing `:facts-keys` membership index. `dl-add-fact!` populates
+  it; `dl-match-positive` walks the body literal's first arg under
+  the current subst — if it's bound to a non-var, look up by
+  `(str arg)` and iterate only the matching subset. chain-25
+  saturation 33s → 18s real (~2x). chain-50 still slow (~120s+)
+  but tractable; next bottleneck is subst dict copies during
+  unification. Differential test bumped to chain-12, semi-only
+  count to chain-25.
+
 - 2026-05-08 — Demo: tag co-occurrence. `(cotagged P T1 T2)` — post
   has both T1 and T2 with T1 != T2 — and `(tag-pair-count T1 T2 N)`
   counting posts per distinct tag pair. Demonstrates count