Remove legacy FSQ emission path

[trashhalo]

Summary

• remove the FSQ head, objective, configuration, checkpoint fields, and chess-era geometry controls
• require an explicit multi-vector emission strategy: concepts/codebook for autoregressive state or QueryBridge for continuous unordered sets
• make the maze reference use ConceptObjective directly and remove examples tied only to the deleted emitter
• preserve continuous single-vector embeddings, QueryBridge emission, codebook/concept state, residual state, CoT emission, KV-cache, and GradCache coverage

Breaking changes

• fsq_dim, fsq_levels, fsq_emit, FSQObjective, label_dims, emb_slots, lam_hard_neg, and ss_sample are removed
• rollout_train_codebook is replaced by rollout_train_state
• multi-vector Trainer runs now require an explicit emission= strategy
• legacy FSQ checkpoints fail with a migration error instead of being silently reinterpreted

Validation

• ruff format --check src tests
• ruff check src tests
• ty check src
• pytest -q (74 passed)