Move Taproot signaling start date earlier to allow more time for miner
preparation and coordination before the March 2027 timeout.
Consensus changes:
- Mainnet activation start: February 14, 2026 (was March 1, 2026)
- Mainnet timeout: March 1, 2027 (unchanged)
- Update nStartTime: 1771027200 (was 1772323200)
- Applied to both mainnet and testnet deployments
This extends the signaling window from 12 months to 12.5 months, providing
miners additional time to upgrade and signal readiness for Taproot activation