docs(rpc): add batch balance snapshot guidance to eth_getBalance

[youssefea]

What changed? Why?

Adds a "Read Balances for Many Addresses" section to the eth_getBalance reference. It explains that integrators must keep their own address list (Base has no organization-level wallet registry), pin every read to one block, batch reads through the Multicall3 preinstall with eth_call, and repeat snapshots for time-based reporting. A note states that such snapshots are RPC reads, not attestations, and that Base does not provide a balance attestation service.

Closes knowledge gap gap-20261006-wallet-enumeration-estate-wide-balance-attestati: "How can a partner enumerate all of its wallets (user smart accounts plus omnibus) and produce time-weighted or attested balances across the whole estate on Base?"

• Gaps log: https://bdocs.cbhq.net/d/base-knowledge-gaps-log
• Linear: DEVREL-859

Notes to reviewers

Primary sources for each fact:

• One address per call, block parameter tags including finalized: docs/base-chain/api-reference/ethereum-json-rpc-api/eth_getBalance.mdx and eth_call.mdx on master.
• Multicall3 preinstall address 0xcA11bde05977b3631167028862bE2a173976CA11: docs/specifications/base-protocol/execution/preinstalls.mdx.
• getEthBalance(address) exists on Multicall3: mds1/multicall3 src/Multicall3.sol.
• Full node keeps a recent block window, archive keeps all history: docs/specifications/node-operators/snapshots.mdx.

Owner confirmation before merge (Base RPC/Node engineering, plus the wallet/data or attestation owner):

• The historical-state wording (full node keeps a recent window; older-block reads need an archive node).
• The statement that Base does not provide a service that attests, audits, or certifies balances across an organization's wallets.

This reuses only the eth_getBalance.mdx part of open PR #2088. That PR also bundles unrelated Base Verify changes, so this PR can replace its eth_getBalance part.

How has it been tested?

• node scripts/lint-mdx.js --files-from=<changed files>: 0 errors. The 2 warnings point at existing code blocks.
• node scripts/validate-docs-structure.js: fails with 2 errors about the /upgrades/denim/eip-8130 redirect. The same 2 errors occur on master without this change.
• Confirmed that every internal link added (eth_getBlockByNumber, eth_call, preinstalls#multicall3, node-operators/snapshots) is in docs/docs.json, and that the ## Multicall3 heading exists.

Screenshots

N/A (text-only change)

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[linear]

DEVREL-859

[cb-heimdall]

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