[Docs] (Disaggregated): the top-level files become short indexes in plain language; the design text moves to design/, the how-to and reference material to guide/

- README, ARCHITECTURE, ROADMAP, MEASUREMENTS and CURRENT_STAGE_PROGRESS are rewritten as indexes
  (84 KB -> 15 KB): what the project is, where it stands, and where each detail lives. ROADMAP is one
  plain sentence per phase; MEASUREMENTS is the five conclusions that matter plus one line per phase;
  CURRENT_STAGE_PROGRESS is the P12 goal, a checklist and the next steps.
- ARCHITECTURE's body moves unchanged into ten design/ files, keeping §1-§17 and appendices A/B, so
  `ARCHITECTURE.md §N` references in code resolve through the table in ARCHITECTURE.md.
- guide/ holds build-and-run, code map, glossary and the engineering discipline (from README and
  ROADMAP), and the device pinning record that used to sit in devices/.
- The open-debt table moves to notes/DEBTS.md; the detailed current-stage tables (with decision IDs,
  the open line items and evidence locations) move into notes/p12/README.md. notes/README.md maps
  every moved path and section.
- The documentation citation lint in test.yml also covers design/ and guide/, which describe the
  current tree as the top-level files did.

.github/workflows/test.yml

@@ -3702,7 +3702,7 @@ jobs:
       - name: Documentation citation lint
         run: |
           shopt -s nullglob
-          documents=(docs/Disaggregated/*.md)
+          documents=(docs/Disaggregated/*.md docs/Disaggregated/design/*.md docs/Disaggregated/guide/*.md)
           if [ ${#documents[@]} -eq 0 ]; then
             echo "no disaggregation documents to check"
             exit 0