[Epic] 5.A GP Advanced Inference and Likelihoods

## Theme
Add the advanced GP inference and observation-model surface that the design docs call out after the Wave 3 baseline.

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

## Parallelism
- Can run in parallel with: #45 and #46
- Blocked by (inside this wave): none
- Must complete before: final advanced-wave polish and cross-cutting docs

## Definition of Done
- Advanced site-based or Newton-style strategies exist beyond VI/CVI.
- Advanced likelihood wrappers cover the main non-scalar or more realistic observation cases.

## Relationships
- Parent wave: #43.
- Blocked by #30.