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.7515.xml

@@ -0,0 +1,14 @@
+<reference anchor="RFC7515" target="https://www.rfc-editor.org/info/rfc7515">
+  <front>
+    <title>JSON Web Signature (JWS)</title>
+    <author fullname="M. Jones" initials="M." surname="Jones"/>
+    <author fullname="J. Bradley" initials="J." surname="Bradley"/>
+    <author fullname="N. Sakimura" initials="N." surname="Sakimura"/>
+    <date month="May" year="2015"/>
+    <abstract>
+      <t>JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification.</t>
+    </abstract>
+  </front>
+  <seriesInfo name="RFC" value="7515"/>
+  <seriesInfo name="DOI" value="10.17487/RFC7515"/>
+</reference>