fix(payment): approve the vault's real merkle charge, not the pool sum

[Nic-dorman]

Problem

A tester's merkle payment fails on every retry with (readable as of rc.3's un-truncation fix):

Payment would fail on-chain: The contract function "payForMerkleTree" reverted with the following signature: 0xfb8f41b2

0xfb8f41b2 decodes to ERC20InsufficientAllowance(address,uint256,uint256) — the vault is authorized to pull less ANT than the payment needs, even though ensureAllowance checks (and tops up) before paying.

Root cause

PaymentVaultV2.payForMerkleTree charges median(winner pool's 16 candidate amounts) × 2^depth (median16 = element 8 of the sorted 16). Our allowance check used maxPoolCost = the largest pool's sum of candidate amounts ≈ 16 × median.

For depth ≥ 5 the real charge exceeds that bound — 2× at depth 5, 8× at depth 7 (a ~460MB / 119-chunk file), 64× at depth 10. Any wallet whose remaining allowance fell between the wrong bound and the real charge skipped the approve and then reverted deterministically. The bug hid behind two masks: shallow trees (charge ≤ pool sum for depth ≤ 4) and fresh standing allowances (1 ANT dwarfs typical charges); a drained allowance plus a deep tree exposes it.

Fix

New merkleMaxCharge(depth, pools) mirrors the contract's math exactly: median at sorted index 8 (the contract's median16 k), × 2^depth, worst case across pools — the winner pool is selected on-chain from sender+timestamp and is unknowable client-side, so the upper bound over pools is the tightest correct answer. ensureAllowance now receives that instead of the pool sum.

For context: upstream evmlib's own wallet sidesteps this by approving U256::MAX; our bounded-allowance design (standing 1 ANT, #191) just needs the bound to be correct.

Testing

• Vitest 16/16 in the payment suite, including: contract-math mirror (1152 = 9 × 2^7 for amounts 1..16), a regression assertion that the new bound exceeds the old pool-sum bound at depth 7, worst-case-across-pools, and empty-pool safety.
• nuxi typecheck clean.

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