[Epic] 5.B GP Inter-Domain Features and Integrations

## Theme
Implement or document the advanced feature/integration/model-pattern surface that connects `pyrox.gp` to inter-domain bases and the broader ecosystem.

## Parent Wave
- Wave epic: `#43`
- Wave label: `wave:5`
- Milestone: `v0.5-advanced-algorithms-and-integrations`

## Parallelism
- Can run in parallel with: #44 and #46
- Blocked by (inside this wave): none
- Must complete before: final advanced-wave polish and cross-cutting docs

## Definition of Done
- Inter-domain inducing-feature support exists.
- Advanced model-pattern and integration docs are present with copied API/demo context where needed.

## Relationships
- Parent wave: #43.
- Blocked by #35.