feat(consolidation): Liquid UTXO consolidation with decoy outputs - #2494

What

Adds Liquid wallet UTXO consolidation: when a wallet accumulates too many
confirmed L-BTC UTXOs, it's offered (via a banner on wallet/send/swap
screens) a guided flow to sweep them into fewer outputs, keeping the wallet
well under the ~256-input confidential-transaction limit before it ever
blocks a real send or swap.

The decoy output, and why it's there

A naive consolidation transaction is an obvious on-chain signature: many
inputs, one output, no counterparty — a many-in/one-out self-transfer. On a
transparent chain that's a privacy leak (heuristics like this are a
well-known way to cluster a wallet's UTXOs together). On Liquid, amounts and
assets are already blinded by confidential transactions, but the shape of
the transaction (input/output count) is still visible.

Each consolidation batch is built with two outputs instead of one: the
real drained amount, plus a second, small "decoy" output back to a
fresh address of the wallet's own. This makes the transaction look like an
ordinary two-output payment rather than a textbook consolidation. Because
Liquid hides amounts, the decoy value itself is never visible to
anyone but the wallet — only the shape of the tx (2 outputs, not 1) is
public, and that shape is indistinguishable from a normal payment.

The decoy UTXO is then frozen (best-effort) so it's never accidentally
swept back into a future consolidation round.

Also included

• Frozen-UTXO enforcement in real sends/swaps — Liquid sends and
  auto-swap executions now explicitly exclude frozen UTXOs (previously only
  the UI bookkeeping did), so a frozen decoy or user-frozen coin can never
  be accidentally spent by a normal transaction.
• Address-reservation datasource — prevents two addresses (e.g. two
  consolidation batches, or a batch running close to another
  address-generating event) from ever being handed out with the same
  index, closing a real address-reuse bug found during testing.
• Sync gap-limit fix — a multi-batch consolidation round now syncs the
  wallet against the actual highest reserved address index afterward,
  closing a real "balance disappeared" bug where funded addresses could
  land outside the default 20-address gap-limit scan.
• buildCustomTx — a new explicit-UTXO-list Liquid transaction builder
  (vs. the old coin-selecting builder), needed so consolidation, frozen-UTXO
  exclusion, and the decoy output can all specify exact inputs/outputs.

Testing

Tested using small amounts of UTXOs.