Bug: Payload generates negative USDC transfer for Beets when fees are zero

[jalbrekt85]

In fee_allocator.py:380-381, all usdc transfer amounts unconditionally subtract 1000 wei as a rounding buffer:

beets_fee_usdc = round(beets_df["amount"] * 1e6) - 1000

When beets fees are zero, this produces -1000. The combined payload then sums v2 and v3 entries to -2000.

Observed in the 2026-02-12 → 2026-02-26 run (PR #298). Manually removed from payloads.

Fix: Skip the transfer (or skip the -1000 subtraction) when the amount is zero.