feat(blockchain): cross-chain ADR-015 deposit reference

[philanton]

Adds an opaque sub-account reference to the deposit API across all chains, building on the EVM binding from #2.

API

VaultDepositor.SubmitDeposit now takes a core.DepositDestination:

type DepositDestination struct {
    Account string   // L1/clearnet account credited
    Ref     [32]byte // ADR-015 opaque sub-account reference; zero = none
}

The reference is side-data — never interpreted on-chain — so deposits are filterable per (Account, Ref). An observer folds it into the account URI; the submit side just carries it.

Per-chain

• EVM — passed to deposit(account, asset, amount, reference).
• SOL — Solana program artifact (IDL + .so) + binding refreshed to the reference-carrying deposit_sol/deposit_spl(account, reference, amount); depositor forwards Ref.
• XRPL — switched from DestinationTag to a ynet-account memo (20-byte account || 32-byte reference), matching the observe-side decoder. New accountMemo + unit test.
• BTC — a plain send has no side-data channel (the account is the per-account deposit address), so a non-zero Ref is rejected.

Scope / breaking

SubmitDeposit's signature changes (pre-1.0). VerifyDeposit unchanged. Tag as v0.1.2 after merge.

Verification

go generate, go build/vet (incl. -tags=integration), go test -count=1 ./... — all green. SOL program id unchanged (98eVpih…); integration deposit flows updated to DepositDestination.

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