roadmap: retire Embedding ABI — M4 (never speced) — speculative infra by feature-acceptance clause 2

M4 (sequence crossing via the List ADT) was struck during its own
/boss brainstorm, in the Step-2/3 Q&A, before any spec — the
"problem mis-framed -> don't ratify a known-unneeded shape into the
roadmap" brainstorm path. No spec, no grounding-check, no planner
handoff.

Why: held against DESIGN.md feature-acceptance criterion clause 2,
M4 removes no redundancy. The shipped M3 export gate is_c_abi_type
(crates/ailang-check/src/lib.rs:1934-1953) is a per-parameter loop
accepting a C scalar OR a single-ctor all-scalar record
independently per param; the forwarder's llvm_scalar maps every
non-scalar Type::Con to ptr (M3-frozen). So (State, Tick) -> State
with both as single-ctor all-scalar records is already
gate-accepted and forwarder-supported today. The minimal
data-server binding is M3 (shipped) + a host-side per-tick loop;
cons-list crossing would add a 2N+1-box-per-chunk host builder and
the deferred flat-array perf debt, with no named consumer (M5's
adapter unrolls each chunk host-side; whole-chunk in-kernel
visibility is semantically void — State threads across calls
regardless of chunk boundaries).

Outcome: M4 retired in docs/roadmap.md (struck entry kept one
cycle for context, never [x] — never shipped); M5 reconciled
(depends on M4 -> M3 + the Tick-coverage todo; adapter unrolls
host-side; friction feeds the host-per-tick-FFI vs. batch-crossing
P2 perf decision); residual recorded as a new [todo] Tick-coverage
on M3 — an E2E + fixture pinning the two-record-param per-tick
(State, Tick) -> State shape (test backfill, no brainstorm).

Rationale + the honest mid-Q&A correction (I had wrongly asserted
M5 needs M4) in
docs/journals/2026-05-18-brainstorm-embedding-abi-m4-retired.md;
INDEX.md mirrors it.

docs/roadmap.md

@@ -304,31 +304,50 @@ work progresses.
     `docs/journals/2026-05-18-audit-embedding-abi-m3.md`;
     `docs/journals/2026-05-18-iter-embedding-abi-m3.tidy.md`.
 
-- [ ] **\[milestone\]** Embedding ABI — M4: sequence crossing via
-  the existing `List` ADT. Host builds a `List Record` per chunk;
-  `(State, List Record) -> State` callable end-to-end. Pure
-  ABI/marshalling, no language change — applies M3's frozen
-  layout repeatedly. Makes the real fold runnable over real
-  chunks. The array-vs-cons-list performance question (1024 RC
-  cons-cells per chunk on the hot path) is deliberately *not* in
-  scope here — capability now, performance as a separate later
-  non-ABI milestone (P2 below), keeping the user's "focus on the
-  ABI, ignore the array gap" cut. Coherent stop:
-  `(State, List M1) -> State` runs.
-  - depends on: Embedding ABI — M3.
-  - context: 2026-05-18 chat.
+- **\[milestone — RETIRED 2026-05-18, never speced\]** Embedding ABI
+  — M4: sequence crossing via the `List` ADT. Struck during its own
+  brainstorm Q&A, before any spec: failed feature-acceptance
+  clause 2. The minimal data-server binding is M3 (shipped) + a
+  host-side per-tick loop over each chunk's records —
+  `(State, Tick) -> State` is *already* gate-accepted and
+  forwarder-supported under M3 (the gate loop accepts each record
+  param independently; the forwarder maps each to `ptr`). Cons-list
+  crossing would *add* a 2N+1-box-per-chunk host builder plus the
+  deferred flat-array perf debt rather than remove redundancy, and
+  builds something no named consumer needs (M5's adapter unrolls
+  the chunk host-side; whole-chunk in-kernel visibility buys nothing
+  — `State` threads across calls regardless of chunk boundaries).
+  Removed (not `[x]` — never shipped); rationale in the journal.
+  Stays one cycle for context, then delete.
+  - context: `docs/journals/2026-05-18-brainstorm-embedding-abi-m4-retired.md`.
+
+- [ ] **\[todo\]** Tick-coverage on M3 — E2E + fixture pinning the
+  two-record-param per-tick crossing `(State, Tick) -> State` on the
+  shipped M3 ABI (host builds `Tick` via `ailang_rc_alloc` per the
+  frozen `make_state` recipe, calls once per tick, threads `State`,
+  alloc==free, both `own`/`borrow`). Zero language change — capability
+  is present today but only E2E-proven for a *single* record param
+  (`State`); this pins the two-record-param shape by fixture. No
+  brainstorm (test backfill). The residual of retired M4 — the actual
+  "minimal data-server binding".
+  - context: `docs/journals/2026-05-18-brainstorm-embedding-abi-m4-retired.md`.
 
 - [ ] **\[milestone\]** Embedding ABI — M5: `ail-embed` adapter +
   `data-server` wiring + thread-swarm backtest (fieldtest
   target). The host-side adapter crate (sole meeting point of
   `data-server` and AILang, per Invariant 1) wires
   `data-server`'s `while let next_chunk` loop to the AILang
-  kernel via the ABI; an actual small backtest runs over a
-  thread swarm. Natural `fieldtest` milestone: existence proof
-  of the whole goal + friction harvest feeding the
-  array-primitive decision. Coherent stop: the goal itself.
-  - depends on: Embedding ABI — M4.
-  - context: 2026-05-18 chat.
+  kernel via the M3 ABI by **unrolling each chunk host-side** into
+  per-tick `(State, Tick) -> State` calls (M4's cons-list crossing
+  was retired — see above; the adapter, not the kernel, owns chunk
+  iteration); an actual small backtest runs over a thread swarm.
+  Natural `fieldtest` milestone: existence proof of the whole goal
+  + friction harvest feeding the host-per-tick-FFI vs. batch-
+  crossing perf decision (the P2 flat-array item). Coherent stop:
+  the goal itself.
+  - depends on: Embedding ABI — M3; Tick-coverage on M3 (todo above).
+  - context: 2026-05-18 chat; M4 retirement
+    `docs/journals/2026-05-18-brainstorm-embedding-abi-m4-retired.md`.
 
 - [x] **\[milestone\]** Heap-`Str` ABI — runtime infrastructure for
   malloc-backed, refcounted `Str` values alongside the existing