Corrects the unsound list-irrefutability rule from 63f7644. Under bounded-max
semantics a List<T,N> value has dynamic length 0..=N, so a fixed-arity
`let [a,b] = xs` (misses every length != 2) and a non-empty-prefix
`let [head, ..rest] = xs` (misses the empty list) are REFUTABLE — only a pure
`[..rest]` (match_list ListCover::Open(0), matches all lengths incl. 0) is
irrefutable. list_refutable_reason now decides off match_list::list_cover (made
pub(crate)) — the single source of list coverage, no forked logic; Exact(k) and
Open(k>0) are E0731 with reasons naming the missed lengths. Records path
(single_arm_covers_all) untouched; ck_lower/codegen unchanged (front-end only).

Goldens flipped to the sound behavior: let_destructure_exact_arity clean->E0731;
let_destructure_list_binds rewritten to a pure [..rest] (the one clean list let);
let_refutable_list_arity reason updated; new let_refutable_list_prefix. Ratchet
held at 112 untrapped. Deleted the dead has_rest/list_size_bound helpers.

Gates: fmt/clippy(-D warnings)/build/test --workspace all exit 0; ck_types 220,
conformance 2. 6 let_pattern #[test]s cover the sound rule exactly.

@@ -0,0 +146,4 @@
+}
+
+impl Teardown {
+    fn new(name: &str) -> Self {