fix(recursion-v2): balance tower transcript buses

[kunxian-xia]

Summary

• wire fork/global transcript ownership for tower and main transcript events
• derive the main transcript claim from the tower replay instead of a placeholder
• gate placeholder batch transcript samples so single-shard recursion v2 e2e balances buses

Tests

• PASS: RUST_MIN_STACK=33554432 CENO_RECURSION_V2_TEST_LOG=logs/e2e-final-fix-tower-e2e-transcript-bus.log scripts/run_e2e_test.sh
• FAIL: RUST_MIN_STACK=33554432 cargo test 'continuation::tests::prover_integration::agg_prover_two_shards' -- --nocapture
  • fails in VmPvsAir row constraints; log saved at logs/e2e-two-shards.log

Stacked on #1372 (base branch: fix/recursion-v2).