iter rpe.1.tidy: subst.rs preserves Type::Fn modes through rebuild

Closes audit-rpe.1's [medium] drift: qualify_local_types_codegen
and apply_subst_to_type at crates/ailang-codegen/src/subst.rs:188 +
:217 both carried the field-spread Type::Fn rebuild shape that
strips param_modes and ret_mode to vec![] / ParamMode::Implicit.
Symmetric to the feb9413 substitute_rigids fix flagged in that
commit's message as a bug class.

Fix: bind param_modes and ret_mode in the destructure, thread
them through unchanged. cargo test --workspace 564/0/3; clippy + doc
zero warnings.

Audit-rpe.1's [nit] dead Emitter.strings field deferred as
known-debt — it cycles over an empty map harmlessly.

Three other field-spread Type::Fn rebuild sites in
ailang-check/src/lib.rs and lift.rs follow the same pattern but
are not currently flagged by failing tests; left in place pending
correctness-reachability verification per the journal Known debt
section.

docs/journals/2026-05-14-iter-rpe.1.tidy.md

@@ -0,0 +1,58 @@
+# iter rpe.1.tidy — Close audit-rpe.1 [medium] drift
+
+**Date:** 2026-05-14
+**Started from:** 973f50b (post-rpe.1 + audit-rpe.1)
+**Status:** DONE
+
+## Summary
+
+Closes the single `[medium]` drift item surfaced by audit-rpe.1:
+`crates/ailang-codegen/src/subst.rs:188,217` — `qualify_local_types_codegen`
+and `apply_subst_to_type` both carried the exact
+`Type::Fn { params, ret, effects, .. }` field-spread that drops
+`param_modes` and `ret_mode` to `vec![]` / `ParamMode::Implicit`. This
+is the same shape the print-leak bugfix at commit feb9413 corrected
+in `crates/ailang-check/src/lib.rs::substitute_rigids`, and the
+feb9413 commit message explicitly flagged any other instance as a
+latent bug in the same class.
+
+Fix: preserve both fields through the rebuild — `param_modes.clone()`
+(it's a `Vec`) and `*ret_mode` (Copy). Symmetric to the feb9413 fix.
+
+The audit's `[nit]` finding (dead `Emitter.strings` field in
+`crates/ailang-codegen/src/lib.rs`, an orphan after `intern_string`
+was removed in rpe.1 Task 4) is left as a follow-up tidy candidate —
+it cycles over an empty map harmlessly and isn't an audit gate.
+
+## Files touched
+
+- `crates/ailang-codegen/src/subst.rs:188` —
+  `qualify_local_types_codegen` Type::Fn arm now binds
+  `param_modes` and `ret_mode` and threads them through.
+- `crates/ailang-codegen/src/subst.rs:217` —
+  `apply_subst_to_type` Type::Fn arm same shape.
+
+## Verification
+
+- `cargo test --workspace` → 564 / 0 / 3 (baseline holds).
+- `cargo clippy --workspace --all-targets` → 0 warnings.
+- `cargo doc --workspace --no-deps` → 0 warnings.
+
+## Concerns
+
+- (none)
+
+## Known debt
+
+- `Emitter.strings` field in `crates/ailang-codegen/src/lib.rs`
+  remains dead. Cycles over empty map; no behavioural impact.
+  Roll into a future opportunistic cleanup.
+- Three other field-spread `Type::Fn` rebuild sites
+  (`crates/ailang-check/src/lib.rs:93` and `:3492`,
+  `crates/ailang-check/src/lift.rs:875`) ALSO drop `param_modes`
+  / `ret_mode` to `vec![]` / `Implicit`. They were not flagged
+  by audit-rpe.1 because no currently-failing test pins them.
+  Leaving conservatively in place — a flag for a future
+  fieldtest or audit cycle. The pattern of field-spread-and-
+  drop is the bug class; whether each site is correctness-
+  reachable is what needs verification before changing.