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

testfiles/mail/gog-roundcube.eml.expect

@@ -0,0 +1,3 @@
+# the text is in the HTML part only; the plain-text part is a one-line placeholder
+Silent Hill: Townfall
+JETZT KAUFEN