Specify the Score Bundle Format, and make tempo an integer

The bundle is documented as a standalone format rather than as a note
between two programs: producers and consumers are generic, the marker
vocabulary is defined musically rather than by what a viewer does with
it, and image properties are stated as guarantees with the reasoning
where it is not obvious. Anything that reads scores can implement it
without knowing this tool exists.

Taking that view changed the substance in three places. Marker types now
carry their musical meaning rather than a UI mapping. The rule against
re-importing became a statement about identity - indices mean something
only within one bundle, so two bundles of a piece are independent
documents. And forward-compatibility rules were added, which a protocol
needs and a handover note did not: ignore unknown fields and marker
types, refuse an unknown version.

Tempo is now an integer, beats per minute, exported as a JSON number and
omitted when blank; the editor accepts digits only. A figure can drive a
metronome or a click track where a verbal marking cannot, and readers do
not agree on what Andante means. This needs the consumer's column
changed from free-form text, which the format document flags.

Every figure in the document comes from a real export.

README.md

@@ -45,6 +45,7 @@ needs a system package.
 |---|---|
 | [CONTEXT.md](CONTEXT.md) | Glossary. What a slice, cut, discard, bundle and song scale actually mean here. Start here. |
 | [docs/spec.md](docs/spec.md) | The specification: pipeline, geometry model, detection, editor, bundle format, and what noteman has to change. |
+| [docs/bundle-format.md](docs/bundle-format.md) | The Score Bundle Format — a standalone specification of the export format, independent of this tool. |
 
 Deferred work is tracked as issues and milestones on the Gitea repo, not in this
 tree.