fix: auto-consolidate fragmented UTXOs when minting DigiDollar

[JohnnyLawDGB]

Summary

• When mintdigidollar fails due to UTXO fragmentation (>400 small UTXOs), the RPC now automatically creates a consolidation transaction and retries the mint
• Fixes the "Too many small UTXOs" error that miners/pool operators encounter when their wallets are full of small mining reward UTXOs
• Response includes consolidation_txid and utxos_consolidated fields when auto-consolidation occurred

Root Cause

SelectCoins in txbuilder.cpp has a MAX_TX_INPUTS = 400 limit to keep transactions under MAX_STANDARD_TX_WEIGHT. When a wallet has many small UTXOs (e.g., 500 x 600 DGB from mining), the top 400 UTXOs may not cover the collateral requirement (e.g., 400 × 600 = 240,000 < 246,000 needed for tier 0).

Fix

In the mintdigidollar RPC handler (src/rpc/digidollar.cpp):

1. First attempt: normal BuildMintTransaction
2. If it fails with "Too many small UTXOs":
   • Create a consolidation tx using the wallet's standard CreateTransaction (which has its own weight-aware coin selection)
   • Broadcast the consolidation tx
   • Re-gather available UTXOs (now includes the unconfirmed consolidated output)
   • Retry BuildMintTransaction with the new UTXO set
3. Both transactions chain in the mempool (parent-child)

Test plan

• Reproduced bug: created wallet with 500 × 600 DGB UTXOs, mintdigidollar 10000 0 failed with "Too many small UTXOs"
• Applied fix, same wallet successfully mints with auto-consolidation
• Response includes consolidation_txid and utxos_consolidated: true
• DD position created correctly (10,000 cents, tier 0, health_ratio 999)
• Wallet UTXO count reduced from 500 → 49 after consolidation + mint
• Normal minting (no fragmentation) unaffected — consolidation path not triggered
• Edge case: wallet with insufficient total DGB returns clear error
• Edge case: wallet with extreme fragmentation (>1700 UTXOs where even consolidation hits weight limits)

🤖 Generated with Claude Code

[DigiSwarm]

Nice work Johnny. Verified this is a real issue — SelectCoins hits MAX_TX_INPUTS = 400 at line 78 of txbuilder.cpp, returns false, and miners with 500+ small UTXOs physically cannot fit enough inputs to meet collateral. The auto-consolidation approach using the wallet's standard CreateTransaction is clean.

Code review notes:

• Consolidation target math is correct (collateral + 10% + 0.1 DGB fee buffer)
• GetNewChangeDestination(BECH32) avoids address reuse ✅
• The unconfirmed consolidation output chains correctly in mempool (parent-child) — low risk on DGB's 15-sec blocks
• Error messages are clear with manual fallback instructions
• Optional response fields (consolidation_txid, utxos_consolidated) are clean UX

Two unchecked edge cases from the test plan:

1. Insufficient total DGB — handled by the consolidationTarget > totalAvailable check, throws clear error ✅
2. Extreme fragmentation (>1700 UTXOs) — CreateTransaction itself would reject oversized consolidation tx. Error message might be confusing but it's a rare edge case.

One minor note: If the consolidation tx gets evicted from mempool before confirmation, the chained mint tx becomes orphaned. Practically zero risk on DigiByte but worth documenting.

Approved. Merging into feature/digidollar-v1. 🚀