feat(raw): raw derive writes an Excel file's sheets as TSV files with a formula list (#202)

Each sheet becomes NN-<sheet>.tsv (RFC 4180, tab-delimited, A1-aligned) in a
<stem>.sheets/ folder beside the .md; sheets with formulas also get a
.formulas.tsv. derived.md is the overview. Adds testfiles/ with a CI test.

Files changed:
- .gitattributes
- CHANGES.md
- VERSION
- docs/dev/development.md
- docs/how-to/ingest-a-source.md
- docs/reference/commands/raw-material-and-uploads.md
- raw/CONTRACT.md
- testfiles/README.md
- tools/CONTRACT.md
- tools/chemenu/commands/raw_cmd.py
- tools/chemenu/filters/__init__.py
- tools/chemenu/filters/archive.py
- tools/chemenu/filters/ooxml.py
- tools/chemenu/tests/test_raw_derive.py
- tools/chemenu/tests/test_raw_derive_testfiles.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.17 - 2026-10-08 - raw pending names only the raw fetch pair as a bundle; a derived document is a folder
+## 9.0.0-beta.18 - 2026-10-09 - raw derive writes an Excel file's sheets as tab-separated files with a formula list; derived.md becomes the overview (#202)
 
 **Author:** Torben Nehmer
 
@@ -47,6 +47,7 @@ place only: `instructions/dev/version-parts.md` § The candidate model.
 - tools/README.md: Asset-Modus beschreibt Entpacken in den eigenen Ordner, Wiederholung und Zurückrollen
 - Gitea-Wiki als generierter Spiegel von docs/: Konverter, Push-Skript, Workflows
 - raw pending names only the raw fetch pair as a bundle; a derived document is a folder
+- raw derive writes an Excel file's sheets as tab-separated files with a formula list; derived.md becomes the overview (#202)
 <!-- /wikitool:bumps -->
 
 ### Tutorials: installation in English, and a first source to a first answer (#191)