maude: witness-path search for puzzle solvers (8 tests, 221 total)

lib/maude/searchpath.sx — mau/search-path returns the shortest sequence of
states from start to goal (the solution moves), mau/search-length its step
count. BFS over all one-step successors, threading the path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/maude-on-sx.md

@@ -109,6 +109,9 @@ The novel substrate stress: equational matching. Pattern `X + Y` against `1 + 2
   a module plus trailing `reduce`/`red`/`rewrite`/`rew TERM .` commands
   (`... in MOD : TERM` qualifier accepted) and execute them, rendering results
   in surface syntax. Runs an idiomatic `.maude` file end-to-end. 6 tests.
+- [x] Witness-path search (`lib/maude/searchpath.sx`) — `mau/search-path` /
+  `mau/search-length` return the shortest sequence of states start..goal (the
+  solution moves), not just yes/no. 8 tests.
 
 ### Phase 8 — Propose `lib/guest/rewriting/`
 - [ ] Extract equational matching engine (the most reusable piece).