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.8693.xml

@@ -0,0 +1,16 @@
+<reference anchor="RFC8693" target="https://www.rfc-editor.org/info/rfc8693">
+  <front>
+    <title>OAuth 2.0 Token Exchange</title>
+    <author fullname="M. Jones" initials="M." surname="Jones"/>
+    <author fullname="A. Nadalin" initials="A." surname="Nadalin"/>
+    <author fullname="B. Campbell" initials="B." role="editor" surname="Campbell"/>
+    <author fullname="J. Bradley" initials="J." surname="Bradley"/>
+    <author fullname="C. Mortimore" initials="C." surname="Mortimore"/>
+    <date month="January" year="2020"/>
+    <abstract>
+      <t>This specification defines a protocol for an HTTP- and JSON-based Security Token Service (STS) by defining how to request and obtain security tokens from OAuth 2.0 authorization servers, including security tokens employing impersonation and delegation.</t>
+    </abstract>
+  </front>
+  <seriesInfo name="RFC" value="8693"/>
+  <seriesInfo name="DOI" value="10.17487/RFC8693"/>
+</reference>