benchmark: add current Feature Selection and Knockoff coverage

[TheHiddenObserver]

Goal

Produce current canonical benchmark coverage for maintained Feature Selection and Knockoff capabilities.

Scope

• replace or rerun pre-June historical evidence with current structured measurements;
• align feature-selection cases, signal sparsity, target FDR, seeds, and external baselines;
• retain selection metrics, failures, timing protocol, and backend identity;
• add parser/schema tests, manifest registration, generated assets, and coverage-matrix update.

Historical April evidence remains audit-only until superseded.

Parent: #100