audit-pd: tidy items + milestone close (prelude-decouple)

Architect drift: 1 Important + 1 Minor fixed inline as audit-pd-tidy.

DESIGN.md §"Roundtrip Invariant" point 4 + §Enforcement asserted
"Eight `.ail.json`-only fixtures" with one being the prelude embed.
Post-pd.3 the count is seven and the embed clause is empty; rewrote
both paragraphs to reflect the post-milestone state with a forward-
pointer to the prelude-decouple resolution.

ail/src/main.rs's migrate-canonical-types subcommand carried a
`local_types.get("prelude")` fallback in `rewrite_type` that was
dead-by-construction after pd.3 retired the synthetic prelude load
in the same subcommand. Removed the or_else arm; replaced with a
multi-line comment naming the milestone and the repopulation path
for any future variant that needs prelude awareness.

Bench: check.py + compile_check.py exit 1 with the established
envelope-noise (15th consecutive observation since audit-cma);
cross_lang.py clean. pd.* iterations are workspace-loader / spec /
filesystem / test-code only — zero codegen / runtime / typecheck
path edited, so the regression cluster cannot be milestone-induced.
Baseline left pristine consistent with all 14 prior decisions.

Roadmap: P0 entry (prelude-decouple, [~]) struck and removed; P0 is
empty again. The pre-milestone P2 entry that this work superseded
("Prelude embed: Form-A as compile-time source") is gone too. Per-
iter journals stay in INDEX.md as chronological context.

Carried debt unchanged: 3 cargo doc warnings on dangling load_workspace
references in core's lib.rs (covered by the existing P2 "Rustdoc
warning sweep" todo).

crates/ail/src/main.rs

@@ -2424,8 +2424,16 @@ fn rewrite_def(
         {
             return;
         }
-        // Bare cross-module — find owner via imports-in-order, with
-        // prelude as last-resort fallback.
+        // Bare cross-module — find owner via imports-in-order. The
+        // implicit-prelude fallback that this helper used to carry
+        // (an `or_else` arm scanning `local_types["prelude"]`) was
+        // dead-by-construction after the prelude-decouple milestone
+        // (2026-05-14): the synthetic prelude entry that the migrate-
+        // canonical-types subcommand used to insert into `modules`
+        // was removed in iter pd.3, so `local_types["prelude"]` is
+        // never populated. Removed in audit-pd-tidy. If a future
+        // migrate variant needs prelude awareness, repopulate
+        // `local_types["prelude"]` explicitly via `parse_prelude`.
         let owner: Option<&str> = import_names
             .iter()
             .map(|s| s.as_str())
@@ -2434,17 +2442,6 @@ fn rewrite_def(
                     .get(*imp)
                     .map(|s| s.contains(name))
                     .unwrap_or(false)
-            })
-            .or_else(|| {
-                if local_types
-                    .get("prelude")
-                    .map(|s| s.contains(name))
-                    .unwrap_or(false)
-                {
-                    Some("prelude")
-                } else {
-                    None
-                }
             });
         if let Some(owner) = owner {
             *name = format!("{owner}.{name}");