v0.19.0

This release explicitly targets Hubuum server v0.0.20.
The server image, Rust 1.88 MSRV, and public feature combinations are unchanged.
Existing callers must follow the 0.19 migration guide
for the breaking Rust API changes below; no server data migration is required.

Changed

• Breaking: resource IDs reject zero and negative values in constructors,
  parsing, and deserialization. Replace primitive conversions with Id::new(value)?
  or Id::try_from(value)?; pass typed IDs to resource queries and relation
  exclusions. ID-bearing resources and required-ID creation bodies no longer
  implement Default. Use checked creation builders or explicit request fields.
• Breaking: low-level resource requests select their resource with a type
  parameter instead of a dummy default instance. Historical relation variants
  now contain their matching typed endpoints. Computed response revisions use
  ComputationRevision. See migration instructions.
• Task discovery validates typed filter state before serializing requests.
  Async and blocking requests share preparation, header validation, and request
  plans. Resource generation resolves each field type once for models and builders.