Keep users funded

[karl-dz]

Users either onboard themselves via Passport and Sentinel flow or are allowed onto the DoubleZero Ledger network via DZF representative.

We should only fund keys that are rent-exempt (an account that has at least 128 * 6960 lamports).

Unlike contributors, users will have at most 2 User accounts it needs to fund for rent exemption. So we should only consider making sure the user balances are topped off with 128 * 6960 + X lamports, where X is some value between 20k and 50k (for 4 to 10 transactions) per funding round

[thijsvanemmerik]

A few specifics worth capturing:

Funder doesn't cover user wallets today

• Simultaneous IBRL + Multicast is the critical case
  Running both tunnels concurrently creates two separate User PDAs from the same wallet. Minimum required:
• • IBRL PDA rent (176 bytes): 2,115,840 lamports
• • Multicast PDA rent (208 bytes): 2,338,560 lamports
• • Wallet rent-exempt minimum: 890,880 lamports
• • Total: ~5,355,280 lamports (0.00536 DZ credits)

The one-time airdrop (~0.00594 DZ credits) barely covers this and gets drained by tx fees over repeated connect/disconnect cycles. Users will inevitably hit this.

Fix
Add automatic funding to user wallets, topping them up to a level that comfortably covers the worst-case simultaneous tunnel scenario with margin. Users should never encounter insufficient funds, the funder should prevent it proactively.

Bad error UX (separate but related)
check_balance() only fails at balance == 0, so users hitting this get a raw Solana runtime error instead of something actionable.