update megaeth v3 to canonical factory (4326 deployment)

The megaeth-mainnet v3 subgraph was indexing a deprecated factory
(0x365de2d31084f7afafd53078eafbf598f29702e8) that only holds 23
low-value pools. All real pools - e.g. WETH/USDm at
0xa42f01610f633d9de29f8d5a5f2782b1570bc580 - are created by the
canonical UniswapV3Factory 0x3a5f0cd7d62452b7f899b2a5758bfa57be0de478
(Uniswap/contracts deployments/4326.md), so v3Pool lookups returned
not-found.

- config.json: factory -> 0x3a5f0cd7..., startblock -> 7009646 (new factory deploy block)
- chain.ts: FACTORY_ADDRESS -> 0x3a5f0cd7...
- chain.ts: STABLE_TOKEN_POOL (WETH/USDT0) -> 0x28bb54b38c8db4606dd50a3d263c1a220b0e4e96
  (USDT0 pool on the new factory; the old 0xd64b667a... is on the deprecated
  factory and would stop being indexed, zeroing ETH/USD pricing)