spec: design-md-rolesplit — DESIGN.md role-split into design/ contracts+models ledger

Role-split the 3020-line docs/DESIGN.md into design/contracts/ (the
hot, test-linked invariant set), design/models/ (one whitepaper per
model), and design/INDEX.md as the sole addressable spine; decision-
records rehomed to docs/journals/. Clean cut (DESIGN.md deleted, no
stub), ###-subsection relocation granularity, build-atomic landing
(design_schema_drift.rs include_str! forces one commit), polymorphic
INDEX links (code-SoT contracts point at source //!, no prose dup),
typed-INDEX anti-regrowth pin (design_index_pin.rs). grounding-check
PASS 13/13 after one re-dispatch (commitment-4 pin-status claim
corrected: Decision-6 :242 audit-trail sentence is pinned by no test,
retired to the decision-record journal, no successor pin by design).

roadmap: DESIGN.md->design/ marked [~] in flight (spec landed).

docs/roadmap.md

@@ -381,7 +381,7 @@ work progresses.
 
 ## P2 — Medium-term
 
-- [ ] **\[milestone\]** DESIGN.md → `design/` — role-split into a
+- [~] **\[milestone\]** DESIGN.md → `design/` — role-split into a
   typed contract ledger + per-model whitepapers, decision-records
   rehomed to `docs/journals/`. `docs/DESIGN.md` is one 2905-line
   file doing three different jobs for three different consumers
@@ -473,10 +473,13 @@ work progresses.
   implementation that closes audits against the current
   DESIGN.md. Brainstorm hard-gate before any plan/code — it
   rewrites enforced invariants and agent contracts.
-  - context: 2026-05-18 chat (DESIGN.md-management dialogue —
-    six-turn convergence: role-conflation diagnosis, three-job
-    split axis, "let the code speak" principle, qmd rejection,
-    blast-radius sequencing). No journal entry yet; pre-brainstorm.
+  - context: spec `docs/specs/2026-05-19-design-md-rolesplit.md`
+    (grounding-check PASS 13/13, one re-dispatch after a corrected
+    commitment-4 pin-status claim); 2026-05-18 chat
+    (DESIGN.md-management dialogue — six-turn convergence:
+    role-conflation diagnosis, three-job split axis, "let the code
+    speak" principle, qmd rejection, blast-radius sequencing).
+    In flight — ABI-arc sequencing blocker cleared by M5 close.
 
 - [ ] **\[milestone\]** Flat array/slice primitive — performance
   follow-up to the Embedding ABI arc, *not* a capability gap.