maude: order-sorted least-sort inference (14 tests, 250 total)

lib/maude/sorts.sx — mau/term-sort computes the least sort of a term (smallest
result sort among op declarations whose arg sorts the actuals satisfy modulo
subsorting); overloaded f(1)=NzNat vs f(s 0)=Nat. mau/has-sort? for
membership-style checks. Answers the plan's order-sorted substrate question.

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

plans/maude-on-sx.md

@@ -112,6 +112,12 @@ The novel substrate stress: equational matching. Pattern `X + Y` against `1 + 2
 - [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.
+- [x] Order-sorted least-sort inference (`lib/maude/sorts.sx`) — `mau/term-sort`
+  computes the least sort of a term: the smallest result sort among the op
+  declarations whose argument sorts the actual args satisfy (modulo subsorting),
+  so an overloaded `f(1)` is `NzNat` but `f(s 0)` is `Nat`. `mau/has-sort?`
+  for membership-style checks. Answers the plan's substrate question — order-
+  sorted signatures fit cleanly. 14 tests.
 - [x] `gather` / parse-time associativity — infix ops parse left (default,
   `(E e)`) or right (`(e E)`) per the gather attr, so cons `_:_ [gather (e E)]`
   reads `a : b : c` as right-nested. Full insertion sort now runs over BARE cons