Add cross-backend verification and update documentation

[vincentcr]

Goal
Establish regression coverage and accurate technical documentation for the supported WSD and legacy HPLIP paths.

Automated coverage

• Discovery fixtures and advertisement grouping.
• Scanner-selection ambiguity.
• Capability normalization.
• Protocol preference and safe fallback.
• Configuration migration.
• Output-format decision table.
• WSD and legacy guest protocol compatibility.

Hardware checkpoints

• Xerox WSD discovery and flatbed scan on the current LAN.
• Xerox three-page feeder behavior.
• HP regression test on the home LAN.
• HP retains its special page-sizing behavior.
• WSD scanning never provisions HPLIP.

Documentation

• Keep CLI help, config.example, and backend technical notes aligned with behavior.
• State that WSD is vendor-neutral while arbitrary legacy support requires an adapter.
• Document scanner and protocol selection flags.
• Record native AirScan as deferred backlog work, not a current capability.
• Leave the full README rewrite to final task Rewrite README for clarity and maintainability #13.

Dependencies
Runs throughout the project and closes after the integration issues.