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 The pre-milestone P2 entry that this work superseded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

@@ -2092,11 +2092,15 @@ Concretely:
    byte-identical to direct `ail parse` of the source `.ail`.
    Pins drift that crate-internal tests cannot see.
 
-4. **Carve-out anchor.** Eight `.ail.json`-only fixtures (seven
-   subject-matter rejection tests + one compile-time-embed for
-   the prelude) survive in the corpus by structural necessity.
-   They participate in their own dedicated rejection-shape or
-   embed-mechanism tests, not in the round-trip gate.
+4. **Carve-out anchor.** Seven `.ail.json`-only fixtures
+   (subject-matter rejection tests) survive in the corpus by
+   structural necessity. They participate in their own dedicated
+   rejection-shape tests, not in the round-trip gate. (The
+   prelude's compile-time-embed carve-out, originally listed
+   here as the eighth fixture, was retired 2026-05-14 by
+   milestone prelude-decouple; the prelude is now embedded as
+   `examples/prelude.ail` in `ailang-surface` and parsed at
+   compile time via `ailang_surface::parse_prelude`.)
 
 Hashing is the consequence the language depends on: BLAKE3 of the
 canonical bytes is well-defined for any `.ail` source via parse-
@@ -2138,7 +2142,7 @@ inherit the gate automatically):
   variants fail compile until the visitor and corpus are extended
   in lockstep.
 - `crates/ailang-core/tests/carve_out_inventory.rs::examples_ail_json_inventory_matches_carve_outs`
-  — exactly the eight named carve-out files exist under
+  — exactly the seven named carve-out files exist under
   `examples/*.ail.json` at any commit. A new `.ail.json` or a
   missing carve-out fails the test.