V6.5: Demo notebook — full validation Graph emitting scores + panels together

## Summary
Capstone notebook: build a single `Graph` that consumes a prediction + reference and emits all V1–V5 scores **plus** all V6 panels as terminal outputs. Demonstrates the D15 motivation in code.

## What to build
`docs/notebooks/validation_v6_capstone.ipynb`:

1. Reuse the Gulf Stream SSH fixture from V1.4 / cmems_ssh_products.
2. Build a `Graph` with two `Input`s (`prediction`, `reference`).
3. Add nodes for: `RMSE`, `SkillByLeadTime`, `FrequencyBandSkill`, `SpreadSkillRatio` (synthetic ensemble), `HeatBudgetResidual` (if V4 fixtures available), `DetectMarineHeatwaves` + `MatchObjects` + object metrics.
4. Add terminal panel nodes from V6.2 / V6.3 / V6.4 — same Graph emits both Figures and DataArrays.
5. Execute → render Figures inline + print scores.

## Acceptance criteria
- [ ] Notebook executes end-to-end.
- [ ] One Graph emits both `xr.Dataset` and `Figure` outputs.
- [ ] Inline figures committed.
- [ ] `xr_toolz_venv` kernel.
- [ ] mkdocs nav under "Validation tutorials".

## Notes
- This is the closing tutorial for the entire validation framework — the page that proves the design decisions (D10, D11, D14, D15) all click together.

Start with the existing V1.4 Gulf Stream SSH fixture and the V6.2, V6.3, and V6.4 panel nodes, then create docs/notebooks/validation_v6_capstone.ipynb using the xr_toolz_venv kernel. Run the notebook end-to-end to confirm one Graph emits both xr.Dataset and Figure outputs, renders figures inline, and prints scores; also verify inline figures are committed and the notebook appears under the mkdocs “Validation tutorials” nav.

Written by the indexing model from the issue text.