Make frontend retries independently configurable

[guyvdbroeck]

Summary

• add one checkpointed frontend retry API with independent preprocessing and vtree policy overrides
• let each retry inherit or override Arjun's SBVA setting and the public vtree specification
• keep preprocessing checkpoints, retry execution, specification validation, and vtree construction inside FrontendSession

Motivation

Retrying preprocessing can expose a structurally different formula, and the vtree policy appropriate for that formula is an independent choice. Applications need to vary either dimension without reconstructing the frontend workflow or relying on a method dedicated to one particular combination.

Design

FrontendSession::retry accepts a FrontendRetryConfig. Its optional arjun_sbva and vtree_spec fields inherit the primary configuration when omitted and apply only to that retry when present. The vtree field uses the same public specification language as RunConfig, including scored portfolio selection and concrete constructors. Every retry begins from the same immutable simplify checkpoint and follows the same preprocessing and construction path.

Validation

• regression test covering inherited policy and independent SBVA/vtree overrides
• all-target tests and doctests
• clippy with warnings denied
• documentation build with warnings denied
• formatting check