Add offline channel display candidates for MSO8000 and SDS3000

[Scaxlibur]

Depends on

• Core typed contract: Add typed scope channel display control WaveBench#15

The candidate tests require a matching Core checkout until the Core change is merged/released.

Summary

• add four-channel display query/configure/restore candidate methods for RIGOL MSO8000
• add the equivalent four-channel candidate methods for SIGLENT SDS3000
• exercise both drivers end-to-end through the real Core ScopeExtensionService with guarded, audited in-memory transports
• cover success, no-op zero-write behavior, invalid/malformed responses, postcondition mismatch, and verified restoration

Production boundary

No real instrument was connected for this work.

Production descriptors, capability lists, profiles, package versions, and user documentation remain unchanged. Both production descriptors still reject scope.channel_display_configure_v2 before instrument I/O. This PR therefore lands offline candidates only; production promotion requires separate model-specific hardware acceptance.

Validation

• MSO8000 package: 397 passed
• SDS3000 package: 86 passed, 1 skipped
• full plugin repository: 1853 passed, 3 skipped
• Ruff: passed
• pip check: passed
• development-environment check: passed
• both package checks / wheel metadata checks: passed
• plugin doctor: passed
• git diff --check: passed