audit(0064/cutover): record shipped drop machinery + retire dead desugar arm

Cycle-close tidy for the #55 cutover (its audit drift-resolution).
Architect drift review found the design ledger out of step with the
codegen that the cutover shipped, plus two debt items; regression green
(733 passed / 0 failed across the workspace).

design/contracts/0008-memory-model.md — the Codegen contract described
the drop-emission *gates* but not the drop machinery the cutover landed.
Added three subsections recording the present state (honesty rule):
- Per-monomorph drop fns for polymorphic ADTs (crates/ailang-codegen/
  src/dropmono.rs: collect_drop_monos / DropAdtMeta, the suffix scheme,
  value-field-skip on the substituted field type). Ratified by
  alloc_rc_value_type_field_is_not_rc_dec_dropped.
- String-literal rep promotion at owned drop sites (StrRep::Static→Heap
  on a Str literal flowing into an Own slot the callee drops; gated
  strictly on Own). Ratified by own_str_literal_arg_is_dropped_exactly_once.
- Same-constructor arm grouping in match desugar (build_chain /
  build_ctor_group bind ctor fields once). Ratified by
  lit_pat_ctor_tail_drop_single_drop + lit_pat_nil_scrutinee_single_drop.

crates/ailang-core/src/desugar.rs — build_chain routes every Ctor-head
arm (including a length-1 run) through build_ctor_group, so
desugar_one_arm's Pattern::Ctor arm was unreachable dead code
duplicating the bind-once lowering. Replaced it with unreachable! naming
the invariant, and corrected two stale doc comments (the module-header
step 4 and the desugar_one_arm doc still described it as handling ctor
arms, and mislabeled the Lit lowering as Term::Match — it is Term::If).
Full workspace stayed green, confirming the arm was dead.

crates/ailang-check/src/linearity.rs — renamed test implicit_fn_is_exempt
→ own_param_consumed_once_is_clean. Post-0062 there is no Implicit and no
exemption; the test pins the ordinary single-Own-consume clean path.

Did the desugar + linearity edits inline rather than via an agent: I had
already loaded build_chain/build_ctor_group/desugar_one_arm to prove the
arm dead, and a sub-agent would have redone the same reading.

crates/ailang-check/src/linearity.rs

@@ -1897,10 +1897,12 @@ mod tests {
         );
     }
 
-    /// All-Implicit fn → check is skipped, no diagnostics, even if the
-    /// body would trigger a use-after-consume under explicit modes.
+    /// A single `Own` param consumed exactly once (the body returns it)
+    /// is clean — no `over-strict-mode`, no use-after-consume. (Pre-0062
+    /// this was the `Implicit`-exemption case; post-cutover there is no
+    /// `Implicit`, so it pins the ordinary single-consume path.)
     #[test]
-    fn implicit_fn_is_exempt() {
+    fn own_param_consumed_once_is_clean() {
         let m = Module {
             schema: ailang_core::SCHEMA.into(),
             name: "t".into(),