docs: Lighter perp DEX on Robinhood Chain API

[buddies2705]

What

New docs page Blockchains → Robinhood → Lighter Perp DEX on Robinhood Chain API (docs/blockchain/robinhood/lighter-perp-dex-api.md) plus its sidebar entry.

Lighter is the perp DEX integrated into Robinhood Wallet since the July 1, 2026 chain mainnet. On Robinhood Chain it runs a full ZkLighter rollup contract (proxy 0x94bab9693ba2f6358507effcbd372b0660afff9d): USDG margin deposits/withdrawals plus the batch lifecycle (~1 batch/min). The page covers:

• What is on-chain vs. inside the Lighter engine — sets expectations that fills/positions/funding are not chain transactions; on-chain data answers money-flow, TVL, and liveness questions
• Contract map and the proxy/implementation gotcha: Log.SmartContract shows rotating implementation facets (last rotation Aug 21), so every query anchors on LogHeader.Address + topic0
• Event reference — topic0 table for all ZkLighter events (Deposit, WithdrawPending, NewPriorityRequest, batch lifecycle, market/asset registry)
• Queries: latest deposits, withdrawal queue, monthly USDG margin flow since launch over dataset: combined, a rollup-liveness subscription, and deposit() calls by selector
• Decoding notes — word layouts for client-side decoding (events currently arrive with empty Signature.Name; ABI registration is tracked internally in BIT-15310, after which the same queries also return decoded Arguments)

Verification

• Every query executed against production streaming.bitquery.io/graphql on Aug 22, 2026 (realtime + combined; combined holds Lighter history back to its June 26 soft start)
• Decode semantics cross-checked on-chain: Deposit.baseAmount matches the ERC-20 USDG leg 1:1; lifetime deposits − withdrawals ≈ the contract's standing USDG balance
• Page rendered locally with docusaurus start — compiles clean, sidebar entry and tables/admonitions render correctly

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