feat: add follower node test support

[andy-thomason]

Summary

Adds support for running non-validator follower nodes in the RBFT testnet.

Changes

--validator-key is now optional (rbft-node)

When the flag is omitted, an ephemeral random key is generated at startup. Since the derived address is not in the on-chain QBFTValidatorSet, the QBFT state machine treats the node as a follower: it only runs upon_new_block and never proposes, prepares, commits, or triggers round changes.

RBFT_ADD_FOLLOWER_AT env var / --add-followers-at CLI arg (rbft-utils)

Comma-separated list of block heights at which to spawn additional non-validator follower nodes. At each configured height, one new node process is launched using the next key slot from nodes.csv (indices starting at num_nodes). The node is not added to the on-chain contract.

make testnet_follower_test target

Generates 6 key slots (4 validators + 2 follower slots), initialises genesis with 4 validators, then starts the testnet with RBFT_ADD_FOLLOWER_AT=5,15 RBFT_EXIT_AFTER_BLOCK=30. Verified passing: both follower nodes spawn, catch up, and reach the exit block in sync with all validators.

Testing

make testnet_follower_test

Output confirms:

• Genesis: 4 initial validators
• Block 5: follower node 4 spawned (role=x in QBFT logs)
• Block 15: follower node 5 spawned
• All 6 nodes reach block 32: [32, 32, 32, 32, 32, 32]