serviceability: expose public fn to calculate access pass airdrop target

[elitegreg]

Summary

• Extract a public, pure airdrop_user_target_lamports(rent, user_airdrop_lamports, multiplier) from airdrop_user_credits in the serviceability program, so the feed oracle can compute the expected access pass airdrop target without duplicating the program's math.
• The function takes &Rent rather than calling Rent::get() internally, so it works both on-chain (caller passes Rent::get()?) and off-chain where Rent is fetched over RPC (getMinimumBalanceForRentExemption or the rent sysvar account).
• airdrop_user_credits now delegates the target calculation to the new function, then subtracts the current balance and transfers the difference. Its signature and behavior are unchanged, so the three existing call sites are untouched.

Testing Verification

• Added a unit test asserting the target equals minimum_balance(AIRDROP_USER_RENT_LAMPORTS_BYTES) + user_airdrop_lamports for multiplier = 1, and that result scaled by N for multiplier = N.
• Existing accesspass and multicast allowlist tests still pass (cargo test -p doublezero-serviceability), confirming the refactor preserves the airdrop transfer behavior.

[juan-malbeclabs]

LGTM