raw derive: a placeholder text/plain no longer displaces the HTML text of a mail (#204)

A mail's text is its plain text unless that is under 10% of the HTML text, then the HTML text
with a warning; one rule for .eml and .msg. The other version of the text in a
multipart/alternative is no longer an attachment.

Files changed:
- CHANGES.md
- VERSION
- docs/reference/commands/raw-material-and-uploads.md
- testfiles/mail/gog-roundcube.eml.expect
- tools/CONTRACT.md
- tools/chemenu/commands/raw_cmd.py
- tools/chemenu/filters/mail.py
- tools/chemenu/filters/msg.py
- tools/chemenu/tests/test_raw_derive.py
- tools/chemenu/tests/test_raw_derive_binary.py

Co-Authored-By: Claude Code <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SnAJ7Z3CpVD3PRbN73QtU2

CHANGES.md

@@ -14,7 +14,7 @@ place only: `instructions/dev/version-parts.md` § The candidate model.
 
 ---
 
-## 9.0.0-beta.19 - 2026-10-09 - testfiles: real documents get short names and a source table, personal data is replaced by same-length filler, and the derive test now compares the sheet TSVs (#203)
+## 9.0.0-beta.20 - 2026-10-09 - raw derive (Mail): ein Platzhalter-Plaintext (unter 10 % des HTML-Texts) verdrängt den HTML-Text nicht mehr, und die zweite Fassung des Mailtexts wird kein Anhang (#204)
 
 **Author:** Torben Nehmer
 
@@ -38,6 +38,7 @@ place only: `instructions/dev/version-parts.md` § The candidate model.
 - raw derive reads PDF-free documents: Office files and mail become a folder with the original and a derived text
 - raw derive unpacks ZIP archives and mails with attachments into a folder
 - raw derive reads PDF and Outlook MSG through lazily imported packages; doctor shows each import filter
+- raw derive (Mail): ein Platzhalter-Plaintext (unter 10 % des HTML-Texts) verdrängt den HTML-Text nicht mehr, und die zweite Fassung des Mailtexts wird kein Anhang (#204)
 
 **Low impact**
 - version bump: Abschlussmeldung nennt Stichpunkt oder Themenabsatz statt pauschal Prosa