feat: INSTALL.md is the English installation tutorial; new tutorial docs/tutorials/first-source-to-first-answer.md with a shipped sample source; README.md links it as its first-ingestion step (#191)

Files changed:
- CHANGES.md
- INSTALL.md
- README.md
- VERSION
- docs/reference/commands/types-instructions-and-docs.md
- docs/tutorials/README.md
- docs/tutorials/first-source-to-first-answer.md
- docs/tutorials/sample-source.md
- instructions/dev/doc-pull-through.md
- tools/CONTRACT.md
- tools/chemenu/commands/docs_verify.py
- tools/chemenu/install_doc.py
- tools/chemenu/tests/test_install_doc.py

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SnAJ7Z3CpVD3PRbN73QtU2

docs/reference/commands/types-instructions-and-docs.md

@@ -400,7 +400,7 @@ tools/wikitool docs prerequisites --apply
 
 **Behaviour**
 
-- Rewrites each `<!-- wikitool:prerequisites -->` region in `INSTALL.md` (tools every platform needs) and `<!-- wikitool:prerequisites-<platform> -->` region (tools only that platform needs) from the manifest: one list item per tool, its label and minimum version. The manifest's reason field stays out - it is English prose, and the region sits in a document that need not be.
+- Rewrites each `<!-- wikitool:prerequisites -->` region in `INSTALL.md` (tools every platform needs) and `<!-- wikitool:prerequisites-<platform> -->` region (tools only that platform needs) from the manifest: one list item per tool, its label and minimum version. The manifest's reason field stays out - it explains a tool to whoever reads the preflight's own output, while the region only says what to install.
 - Never places a region: where a list belongs in the human guide is that guide's own decision. A region the manifest calls for but the file lacks is an error naming the marker pair to add.
 - Dry-run by default (says whether the file would change); `--apply` writes.
 - `docs verify` checks the result stays current.