Bifrost: Hold vault lock through broadcast & add RPC context deadlines

Supersedes !4448 (closed).

This is a replacement of !4448 (closed), which was opened from a fork branch. Scope/changes are the same. Please continue review here.

Summary

This MR improves UTXO signing safety and RPC resilience, and aligns ZEC/BTC RPC client interfaces so one shared client abstraction can be used.

Problem

• Vault-level locking did not consistently cover the full sign -> broadcast window, which can allow concurrent UTXO selection/broadcast races.
• UTXO RPC wrapper calls did not consistently enforce context deadlines.
• rpc.Client and zec.Client had mismatched interfaces, forcing split fields and chain-specific branching.
• Simulation code still used the old rpc.NewClient signature.
• UTXO fee state (lastFeeRate, minRelayFeeSats) was shared across scanner/consolidation paths without synchronized access.
• "already in block chain" broadcast handling did not mark signer cache as signed on that branch.

Changes

• Hold vault lock through broadcast in UTXO consolidate flow and signer flow (via lock-capability interface) to eliminate local double-spend risk on concurrent work.
• Add timeout-aware context handling to UTXO RPC wrapper calls (Call and batch paths).
• Pass configured HTTP timeout into UTXO RPC client creation.
• Add CallContext and BatchCallContext to zec.Client.
• Introduce a shared rpcClient interface (Call, BatchCall, CallContext, BatchCallContext) and use a single client field for both standard RPC and ZEC RPC implementations.
• Update simulation UTXO constructor call to match new RPC client signature.

Additional changes in commit a5dd99b5

• Convert UTXO fee fields to atomics and update all access sites to Load/Store:
  • minRelayFeeSats
  • lastFeeRate
• Refactor consolidate flow:
  • Extract per-vault helper consolidateVaultUTXOs(...)
  • Return wrapped errors from helper
  • Log once at caller with vault context
• In consolidate amount building:
  • Use getUtxoToSpend(..., 0) as count-based probe
  • Sum via per-UTXO sats conversion (btcutil.NewAmount) to avoid float accumulation drift
• In BroadcastTx, when RPC returns "already in block chain", mark signer cache as signed before returning success.
• Add a short code comment documenting why sats-per-UTXO summation is used.

Impact

• Safer concurrent signing/broadcast behavior for UTXO vaults.
• Better failure behavior on slow/stuck RPC calls via deadlines.
• Less duplicated chain-conditional RPC code and cleaner client abstraction.
• Removes unsynchronized shared fee-field access between goroutines.
• Cleaner consolidate error flow (single log location with wrapped context).
• Avoids potential rounding drift in consolidation total amount computation.
• Ensures already-broadcast transactions are handled consistently by always recording them in the signer cache.

Test plan

• make test passes
• make lint passes
• make test-simulation passes