iter 24.tidy: close 5 actionable drift items from audit-24

Documents the three iter-24.3 strengthenings as load-bearing
invariants and tightens two error-handling sites:

T1: DESIGN.md gains new subsection §Cross-module references in
synthesised bodies (between Resolution-and-monomorphisation and
Defaults-and-superclasses) documenting three invariants installed
in iter 24.3 — (1) MonoTarget::FreeFn::type_args carries canonical
types post-collection via normalize_type_for_lookup; (2) post-mono
synthesised body cross-module refs may bypass the source template's
import_map (codegen falls back to module_user_fns / module_def_ail_types);
(3) FreeFnCall synth pushes one ResidualConstraint per declared
forall-constraint with rigid vars substituted by fresh metavars.

T2: codegen_import_map_fallback_pin.rs (integration test) asserts
the synthesised prelude.print__<IntBox> body references
show_user_adt.show__<IntBox> AND prelude module's imports do not
contain show_user_adt — proving the cross-module ref bypasses
import_map at codegen.

T3: polyfn_dot_qualified_branch_pin.rs (integration test) asserts
bare-name print f (f : Int -> Int) fires exactly one no-instance
diagnostic at typecheck with zero unknown-variable diagnostics —
proving the bare-name resolution reaches the dot-qualified synth
branch where the constraint-residual push fires.

T4: check/lib.rs:2858 unwrap_or_default() replaced with .expect()
carrying the registry-coherence message — class_methods index
drift now surfaces explicitly rather than rendering NoInstance
with an empty method name.

T5: mono.rs gains apply_subst_and_normalize helper (Option<Type>
return) extracted from two byte-identical call sites at
collect_mono_targets and collect_residuals_ordered. Each call site
retains its own rigid-var / unit-default policy in the None arm
(site 1: rigid → has_rigid+break, unbound → Type::unit; site 2:
non-concrete → Type::unit). Byte-identity invariant on mono-symbol
hashes enforced by construction.

Tests: 558 passed (was 556 + 2 new pins). No production semantic
change — pure documentation + test pin + error-handling tightening
+ helper refactor. bench/cross_lang exit 0; bench/compile_check +
bench/check exit 0 this run (latency.implicit_at_rc / latency.explicit_at_rc /
bench_list_sum.bump_s noise envelope unobserved, lineage continues
at 10th consecutive observation without firing this run).

bench/orchestrator-stats/2026-05-13-iter-24.tidy.json

@@ -0,0 +1,19 @@
+{
+  "iter_id": "24.tidy",
+  "date": "2026-05-13",
+  "mode": "standard",
+  "outcome": "DONE",
+  "tasks_total": 6,
+  "tasks_completed": 6,
+  "reloops_per_task": {
+    "1": 0,
+    "2": 0,
+    "3": 0,
+    "4": 0,
+    "5": 0,
+    "6": 0
+  },
+  "review_loops_spec": 0,
+  "review_loops_quality": 0,
+  "blocked_reason": null
+}