Merge loops/minikanren into architecture: full miniKanren-on-SX library

Squash merge of 76 commits from loops/minikanren. Adds lib/minikanren/
— a complete miniKanren-on-SX implementation built on top of
lib/guest/match.sx, validating the lib-guest unify-and-match kit as
intended.

Modules (20 .sx files, ~1700 LOC):
  unify, stream, goals, fresh, conde, condu, conda, run, relations,
  peano, intarith, project, nafc, matche, fd, queens, defrel, clpfd,
  tabling

Phases 1–5 fully done (core miniKanren API, all classic relations,
matche, conda, project, nafc).

Phase 6 — native CLP(FD): domain primitives, fd-in / fd-eq / fd-neq /
fd-lt / fd-lte / fd-plus / fd-times / fd-distinct / fd-label, with
constraint reactivation iterating to fixed point. N-queens via FD:
4-queens 2 solutions, 5-queens 10 solutions (vs naive timeout past N=4).

Phase 7 — naive ground-arg tabling: table-1 / table-2 / table-3.
Fibonacci canary: tab-fib(25) = 75025 in seconds, naive fib(25) times
out at 60s. Ackermann via table-3: A(3,3) = 61.

71 test files, 644+ tests passing across the suite. Producer/consumer
SLG (cyclic patho, mutual recursion) deferred — research-grade work.

The lib-guest validation experiment is conclusive: lib/minikanren/
unify.sx adds ~50 lines of local logic (custom cfg, deep walk*, fresh
counter) over lib/guest/match.sx's ~100-line kit. The kit earns its
keep ~3× by line count.

lib/minikanren/tests/pythag.sx

@@ -0,0 +1,36 @@
+;; lib/minikanren/tests/pythag.sx — Pythagorean triple search.
+;;
+;; Uses ino + intarith goals to find triples (a, b, c) with
+;; a, b, c ∈ [1..N], a ≤ b, a² + b² = c². With intarith escapes
+;; the search runs at host-arithmetic speed.
+
+(define
+  digits-1-10
+  (list
+    1
+    2
+    3
+    4
+    5
+    6
+    7
+    8
+    9
+    10))
+
+(mk-test
+  "pythag-triples-1-to-10"
+  (let
+    ((triples (run* q (fresh (a b c a-sq b-sq sum c-sq) (ino a digits-1-10) (ino b digits-1-10) (ino c digits-1-10) (lteo-i a b) (*o-i a a a-sq) (*o-i b b b-sq) (*o-i c c c-sq) (pluso-i a-sq b-sq sum) (== sum c-sq) (== q (list a b c))))))
+    (and
+      (= (len triples) 2)
+      (and
+        (some
+          (fn (t) (= t (list 3 4 5)))
+          triples)
+        (some
+          (fn (t) (= t (list 6 8 10)))
+          triples))))
+  true)
+
+(mk-tests-run!)