Add internal canonical ROC v2 spec builder - #373

Goal

Add the first internal-only canonical rtichoke_viz v2 builder for ROC without switching production rendering away from the current v1/Plotly paths.

Boundary

The builder consumes:

existing statistical calculations -> _build_evaluation_metadata() -> canonical ROC-v2 spec

No statistical quantity is recomputed in the visualization layer.

Semantics

• reference_group is used only as the compatibility join key between existing performance rows and semantic evaluation metadata.
• shared outcome arrays preserve model identities and one shared population;
• keyed populations preserve distinct population identities while model identity remains unknown;
• evaluation IDs and series IDs are deterministic ordinals and do not encode compatibility group names, display labels, prevalence, coordinates, or colors;
• seriesId is the geometric identity;
• display label/group/role remain presentation metadata;
• ROC random-guess identity is emitted as a global reference.

Tests

Characterization coverage includes:

• one model × one population;
• multiple models × shared population;
• keyed/multiple populations with unknown model identity;
• renamed compatibility groups retaining the same deterministic evaluation/series ID convention.

Scope

• internal-only builder;
• no public API changes;
• no production rendering switch;
• no statistical changes;
• no changes to vendored assets in this PR.

Dependency status

The immutable rtichoke_viz v0.2.0 vendoring change is now merged on main via #374, and this branch has been synchronized with that main.