Clean up continuation AIR setup

[MauroToscano]

Summary

• remove the unused first-epoch register-init branch from continuation AIR construction
• make epoch_touched_cells test-only instead of exposing it from production builds
• clean stale comments that referred to the old helper name / is_first binding

Tests

• cargo test -p lambda-vm-prover test_local_to_global_traces_from_real_execution -- --nocapture
• cargo test -p lambda-vm-prover test_epoch_proof_commits_l2g -- --nocapture
• cargo test -p lambda-vm-prover test_continuation_pipeline_end_to_end -- --nocapture
• cargo test -p lambda-vm-prover continuation::tests -- --nocapture (initial run: 14 passed, 1 fixture-missing failure)
• cargo test -p lambda-vm-prover continuation::tests::test_split_verify_rejects_tampered_private_input_genesis -- --nocapture
• make lint