maude: gather / parse-time associativity for cons lists (7 tests, 236 total)

Infix ops parse left (default / gather (E e)) or right (gather (e E)) per the
gather attribute, so _:_ [gather (e E)] reads a : b : c as right-nested. Full
insertion sort now runs over bare cons lists with no parentheses.

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

plans/maude-on-sx.md

@@ -112,6 +112,10 @@ 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] `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
+  lists (no parens). 7 tests.
 - [x] `owise` equations — parser now reads trailing eq attributes
   (`eq L = R [owise] .`), `mau/split-attrs`; `mau/crewrite-top` is two-pass
   (ordinary equations first, owise last), so an owise catch-all fires only when