[openhuman#7523] tinydocs: bus helper to unify presentation spec types #40

Context: openhuman master epic tinyhumansai/openhuman#7523 moves tiny* implementation out of openhuman-core. Parent epic for this module: tinyhumansai/openhuman#7527. Related: tinydocs#31.

OpenHuman's tools/impl/presentation/types.rs redeclares SlideSpec, SlideImage and SlideImageSource next to tinydocs_bus::spec::presentation. The host only needs them to add artifact/workspace-file image sources, which are security policy and stay in the host.

• tinydocs-bus: add a SlideSpec constructor taking text fields plus resolved SlideImage values and a serde round-trip test pinning the wire shape, so the host maps its tool input to the bus type without redeclaring it; no new image-reference variant (artifact/file sources are host policy), no contract version bump; release the module and bump the pin (tinydocs-bus CONTRACT_VERSION stays 4) (openhuman#7578)