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

docs/how-to/ingest-a-source.md

@@ -20,7 +20,8 @@ pick a directory:
   Outlook `.msg`) or a ZIP archive - is
   put into `incoming/` as it is. `tools/wikitool raw derive incoming/<file>` turns it into a folder with the
   untouched original and a readable text beside it, with a header saying how it was made. The
-  folder is accepted whole; the agent runs this itself when it meets such a file. A ZIP is
+  folder is accepted whole; the agent runs this itself when it meets such a file. An Excel file's
+  sheets land beside the text as tab-separated files, with their formulas listed apart. A ZIP is
   unpacked into a folder instead, and a mail's attachments land beside it. A scanned PDF has no
   text to derive - the tool warns, and there is no OCR.
 - **A page behind a paywall or a login** is saved from your browser into `incoming/`, HTML only. The