[mainnet] Register subnet on Bittensor mainnet and enable weight-setting

[Dipraise1]

Context

Phase 2 hardening prerequisites are largely shipped: erasure coding (b73a5012), multi-validator consensus (bfda87b7), slash registry + staking gate (d933842e), stake-weighted replication + miner reputation. The subnet still runs on testnet (netuid 450).

Tasks

• Register the subnet on Bittensor mainnet (recycle cost + hotkeys)
• Deploy validator with ENGRAM_ENV=mainnet (REQUIRE_HOTKEY_SIG enforced)
• Verify weight-setting lands on mainnet
• Confirm staking gate and slash registry behave with real stake
• Update README/docs with the mainnet netuid

From ROADMAP.md — Phase 2 'Mainnet registration'.