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>

lib/maude/scoreboard.md

@@ -1,6 +1,6 @@
 # maude scoreboard
 
-**229 / 229 passing** (0 failure(s)).
+**236 / 236 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -9,6 +9,7 @@
 | matching | 28 | 28 | ok |
 | conditional | 19 | 19 | ok |
 | owise | 8 | 8 | ok |
+| gather | 7 | 7 | ok |
 | rewrite | 21 | 21 | ok |
 | searchpath | 8 | 8 | ok |
 | strategy | 19 | 19 | ok |