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 The pre-milestone P2 entry that this work superseded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/roadmap.md

@@ -35,17 +35,8 @@ work progresses.
 
 ## P0 — In flight
 
-- [~] **\[milestone\]** Prelude / core decoupling — retire
-  `examples/prelude.ail.json` + remove all literal-`"prelude"` strings
-  from `crates/ailang-core/src/`. Three iters: pd.1 (core API split,
-  shipped 2026-05-14), pd.2 (surface assumes ownership of prelude
-  embed + cross-form-identity preflight), pd.3 (delete the JSON +
-  carve-out cleanup).
-  - context: `docs/specs/2026-05-14-prelude-decouple.md`; per-iter
-    journals 2026-05-14-iter-pd.1 (and pd.2 / pd.3 to follow). Reframed
-    from the original "Prelude embed: Form-A as compile-time source"
-    P2 entry to widen scope from pure-embed-swap to full prelude-
-    decoupling of `ailang-core` (β.2 loader-split shape).
+_(empty — prelude-decouple milestone closed 2026-05-14, audit-pd
+clean. Pick the next milestone from P2.)_
 
 ## P1 — Next