[epic] document MirrorNodeAccountBalanceQuery as the sdk-native migration path for AccountBalanceQuery

[theekrystallee]

background

MirrorNodeAccountBalanceQuery is an sdk-native replacement for the deprecated AccountBalanceQuery. it reads an account's hbar balance from the mirror node rest api (GET /api/v1/balances) while keeping the sdk query interface (.setAccountId().execute(client)), with automatic retries and no query payment. verified against hiero-ledger/hiero-sdk-js source at tag v2.87.0 and the published @hiero-ledger/sdk npm package.

availability (verified against the sdk repos, not a roadmap claim):

sdk	version	status
javascript / typescript (hiero-sdk-js)	v2.87.0+	available now
java (hiero-sdk-java)	—	in progress
go (hiero-sdk-go)	—	in progress
rust (hiero-sdk-rust)	—	no implementation, no announced timeline

documentation requirements

• cover every language tab the pages carry. get-balance, tokens/get-balance, and queries document javascript, java, go, and rust. rust has no replacement and no timeline (verified against hiero-sdk-rust source), so rust readers are routed to the mirror node rest api explicitly and never told "coming soon."
• HBAR only. MirrorNodeAccountBalance returns a single hbars field. it is not a token-balance query; tokens/get-balance.mdx must keep readers on the rest token-balances endpoint (clarify that MirrorNodeAccountBalanceQuery does not cover token balances (native/tokens/get-balance.mdx) #722).
• terminology. "public key alias" is a retired term enforced by the terminology check; use "evm address, or account alias."

out of scope

• an in-repo "migrating from AccountBalanceQuery" guide (currently external).
• reference/protobuf/crypto/cryptogetaccountbalance.mdx (protocol-level, unaffected).