ZK activation at height 997 (early blocks have no commitment)

Found a fun chain-init bug. The ZK proof verification path was being triggered on every block from genesis, but the first ~1000 blocks were minted before the shielded-pool program was deployed. The verifier kept trying to read a commitment tree that didn’t exist; transactions silently dropped.

Fix: hardcode ZK_ACTIVATION_HEIGHT = 997 so blocks 0..996 skip ZK verification entirely (legacy-validation-only) and 997+ run the full pipeline. Bitcoin does this for every fork — SegWit, Taproot, witness-v2 — and now we do too. Soft-fork-able once we’re on chain.