Implement native macOS scan acquisition

[vincentcr]

Goal
Extend the ImageCaptureCore helper from discovery into a production native acquisition backend.

Scope

• Open and close scanner sessions.
• Select flatbed or feeder.
• Configure resolution, color mode, and page dimensions where supported.
• Scan one or multiple pages.
• Emit progress and page-result events.
• Support cancellation.
• Normalize empty-feeder, jam, busy, and unsupported-option errors.

Acceptance criteria

• Flatbed and feeder jobs return page files without starting the VM.
• Cancellation leaves the scanner available to a subsequent run.
• Unsupported requested capabilities fail before paper moves.
• The helper protocol has automated tests.

Dependencies
Blocked by the discovery spike and normalized backend contract issues.

[vincentcr]

Descoped from the current scanner-backends refactor because no eSCL/AirScan scanner is available for positive discovery and acquisition testing. The ImageCaptureCore spike remains in the repository, and this issue can be reopened when compatible hardware is available. Current work will proceed with the physically verifiable WSD and legacy HPLIP paths.

[vincentcr]

Reopened and moved conceptually to the backlog. Native ImageCaptureCore acquisition remains worthwhile, but is deferred until an eSCL/AirScan scanner is available for positive hardware validation. It is no longer a dependency of the current WSD plus legacy routing work.