Tasks: T1 DESIGN.md new subsection §Cross-module references in
synthesised bodies documenting three iter-24.3 strengthenings
(canonical-form type_args, post-mono cross-module-ref import_map-
bypass, FreeFnCall constraint-residual push). T2 codegen import_map-
fallback pin (integration-style in crates/ail/tests/). T3 bare-name
poly-fn dot-qualified-branch invariant pin (asserts NoInstance fires
at typecheck not codegen). T4 replace unwrap_or_default() at
check/lib.rs:2852-2858 with expect(class_methods registry coherence).
T5 extract apply_subst_and_normalize helper from duplicated mono.rs
sites at :685-714 + :1284-1303. T6 integration + bench verification.
Boss decisions in Pre-flight section: T1 placement = new subsection
between DESIGN.md:1693 and :1695 (anchorable block beats append-in-
place); T2+T3 = integration-style tests (canonical xmod-test pattern,
not in-crate codegen); T5 helper extracts common concrete-resolved
arm only, returns Option<Type> so each call site keeps its rigid-
var / unit-default policy explicit.
