docs(P00): research findings — v1.27 staleness inventory + backfill sources

NFR milestone, no new domain. Research is a codebase-grounded inventory:
- 11 files to archive (4 pre-execution v1.26 artifacts + 3 phase
  verifications/review + 1 evidence snapshot + 3 durable refs superseded
  by STATE.md/NORTH_STAR/archive + 1 consumer ROADMAP).
- 12 files kept active (no-edit: live code paths, durable refs).
- 3 files kept active (fix-only: PROJECT.md, ROADMAP.md, archive/README.md).
- STATE.md backfill sources: regression_verify.py, registry.json,
  REQUIREMENTS traceability, CHECKPOINT, git log, PROJECT decisions.
- Persona roster: lead-developer only (docs/chore milestone).
- 4 risks, all Low-Medium with documented mitigations.

---ci---
project: acdl
phase: 0
milestone: v1.27
status: research
---ci---

.ciagent/CHECKPOINT.json

@@ -1,6 +1,6 @@
 {
   "phase": 0,
-  "stage": "specify",
+  "stage": "clarify",
   "milestone": "v1.27",
   "phase_role": "pre_execution",
   "attempts": 0,