feat: unified drafts/ structure with PDF outputs for ACT and ECT

Both drafts now live in workspace/drafts/ as siblings:
  drafts/
  ├── act/                       (ACT -01, native to parent repo)
  │   ├── draft-nennemann-act-01.md     kramdown-rfc source
  │   ├── draft-nennemann-act-01.{xml,txt,html,pdf}
  │   ├── .refcache/             bibxml cache
  │   └── build.sh
  ├── ietf-wimse-ect/            (ECT -02, submodule, PDF added)
  │   └── ...
  └── README-pdf.md              PDF toolchain docs

ACT kramdown-rfc conversion:
- full YAML frontmatter (title, author, refs)
- section structure matches kramdown-rfc conventions
- {{REF}} citation syntax, auto-numbered sections
- references auto-built from normative/informative blocks
- removed manual TOC (kramdown-rfc generates)
- builds cleanly: 133K XML, 89K TXT, 208K HTML, 167K PDF

PDF toolchain:
- xml2rfc --pdf via weasyprint<60 + pydyf<0.10 injected into xml2rfc pipx venv
- both build.sh scripts now produce PDF as Step 4
- README-pdf.md documents the setup for new machines

Submodule: bump ietf-wimse-ect pointer for build.sh PDF step

workspace/drafts/act/.refcache/reference.RFC.7518.xml

@@ -0,0 +1,12 @@
+<reference anchor="RFC7518" target="https://www.rfc-editor.org/info/rfc7518">
+  <front>
+    <title>JSON Web Algorithms (JWA)</title>
+    <author fullname="M. Jones" initials="M." surname="Jones"/>
+    <date month="May" year="2015"/>
+    <abstract>
+      <t>This specification registers cryptographic algorithms and identifiers to be used with the JSON Web Signature (JWS), JSON Web Encryption (JWE), and JSON Web Key (JWK) specifications. It defines several IANA registries for these identifiers.</t>
+    </abstract>
+  </front>
+  <seriesInfo name="RFC" value="7518"/>
+  <seriesInfo name="DOI" value="10.17487/RFC7518"/>
+</reference>