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 The pre-milestone P2 entry that this work superseded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).

docs/journals/INDEX.md

@@ -69,3 +69,4 @@
 - 2026-05-14 — iter pd.1: core API split — load_modules_with + build_workspace + implicit_imports threading; load_workspace_with kept as 3-line shim so surface unchanged; production literal-"prelude" count in workspace.rs dropped 8→4 → 2026-05-14-iter-pd.1.md
 - 2026-05-14 — iter pd.2: surface owns prelude embed (PRELUDE_AIL + parse_prelude in ailang-surface); pd.1 shim load_workspace_with retired along with PRELUDE_JSON / load_prelude / load_one; cross-form-identity preflight PASSED at module_hash 3abe0d3fa3c11c99; production literal-"prelude" in core/src/ now zero → 2026-05-14-iter-pd.2.md
 - 2026-05-14 — iter pd.3: prelude.ail.json deleted from working tree; cross-form-identity preflight + supporting bytes deleted from prelude_module_hash_pin (purpose discharged); migrate-bare-cross-module-refs defensive include + lockstep skip-branch deleted; carve_out_inventory.rs §C4(b) row dropped (8→7); form-a-default-authoring spec §C4(b) RETIRED status marker added; new prelude_decouple_carve_out_pin.rs asserts JSON file does not exist; milestone prelude-decouple closed → 2026-05-14-iter-pd.3.md
+- 2026-05-14 — audit-pd: milestone close (prelude-decouple) — architect drift fixed inline as audit-pd-tidy (DESIGN.md §"Roundtrip Invariant" carve-out count 8→7 + main.rs migrate-subcommand dead prelude fallback removed); bench mixed (check.py + compile_check.py established noise envelope, 15th consecutive observation, baseline pristine; cross_lang clean) → 2026-05-14-audit-pd.md