Add design docs: glossary, ADR 0001, and project scaffolding

Records the outcome of the design review that reworked the original
handoff notes:

- CONTEXT.md — ubiquitous language for the slicer (slice, cut, discard,
  content rectangle, trim, song scale, bundle, project, jump source)
- docs/adr/0001 — the slicer owns all image processing and the bundle is
  the only channel to noteman; noteman's sharp pipeline goes away
- pyproject renamed to noteman-slicer
- .gitignore keeps scores and bundles out of the repo (copyright, size)

.gitignore

@@ -0,0 +1,8 @@
+__pycache__/
+*.py[cod]
+.venv/
+
+# Scores and their working state — copyright, and multi-MB
+*.pdf
+*.zip
+*.slicer.json