feat: add ITransactionTransformer for Liquid network support

[dangershony]

Summary

Adds an abstraction layer for network-specific transaction transformations, enabling Liquid (Elements) network support alongside Bitcoin.

New files

• *\ITransactionTransformer* — interface for wrapping P2WPKH, Taproot, and P2WSH transactions for the target network. Also provides \GetSighashTransaction\ / \GetSighashSpentOutputs\ for computing sighashes on the correct network representation.

• *\BitcoinTransactionTransformer* — no-op implementation for Bitcoin. All methods return their inputs unchanged.

• *\LiquidTransactionTransformer* — Elements implementation that rebuilds transactions with explicit L-BTC asset tags, adds an explicit fee output, and re-signs using the Liquid network parameters.

Modified files

• *\TransactionInfo* — added \TransactionHex\ and \TransactionId\ properties so network-specific representations (e.g. Elements hex) can be carried alongside the Blockcore \Transaction\ object.

[dangershony]

Integration assessment ? what's left to make Liquid work end-to-end

This PR adds the cryptographic conversion layer only; nothing calls the transformer yet. Here's an assessment of the remaining work to get Liquid working on mainnet.

Already done on main

• AngorNetwork.Liquid() exists (src/shared/Angor.Shared/Networks/AngorNetwork.cs) wrapping NBitcoin's Liquid mainnet
• Network selection UI (Avalonia, design app, webapp) already lists Liquid
• Per-network indexer/explorer config with Liquid defaults (https://liquid.angor.online) and genesis-hash validation
• Unblinded ex1 address generation works via NBitcoin's Liquid network object

Remaining code work

Task	Notes
Wire the transformer into ~10 sign/broadcast sites	WalletOperations.cs (4 spots), InvestorTransactionActions.cs, FounderTransactionActions.cs, SeederTransactionActions.cs, SpendingTransactionBuilder.cs, InvestmentTransactionBuilder.cs + per-network DI registration ? largest task
Broadcast paths must send TransactionInfo.TransactionHex (Elements hex) instead of .ToHex()	WalletOperations.cs, MempoolSpaceIndexerApi.PublishTransactionAsync
Fee logic	Liquid fees ~0.1 sat/vB, fee is an explicit output, Elements txs are larger than the Bitcoin vsize math assumes
L-BTC asset filtering in client-side Esplora parsing	Elements vouts carry an asset field; without filtering, any non-L-BTC asset sent to a wallet address would be counted as sats
Tests	No transformer or Liquid tests exist yet
Confidential lq1 addresses / blinded UTXOs	Out of scope here ? separate, larger phase

Infrastructure

Since Angor project scanning is now computed client-side against generic Esplora endpoints, no custom indexer is needed ? just:

1. An Elements node (mainnet for prod; liquidtestnet or local Elements regtest for dev ? the docker/boltz/ stack already includes Elements regtest)
2. A stock Esplora-Elements instance at liquid.angor.online (doubles as the explorer)

Risk concentration

The riskiest part is the Elements taproot sighash logic in this PR ? it needs validation by broadcasting real invest ? recover ? founder-spend ? penalty flows on liquidtestnet before mainnet.