* s3vectors: bound body buffering before authentication
* docs: include vector engine installation and source builds
* s3vectors: prevent MIME sniffing of JSON error responses
* docs: load vector Cargo configuration when building from source
* docs: note the native protoc requirement for Windows vector builds
* docs: make vector source checkout and working directory explicit
* ci: handle deb822 sources during Ubuntu mirror failover
* ci: bound Ubuntu mirror attempts and reject partial index updates
