reserve denies a zero-cost estimate, so a genuinely free (zero-priced) model can't transit the gate

[JumpTechCode]

reserve raises NonPositiveEstimate (422) when the worst-case estimate rounds to 0. Because ceil_micro keeps any positive worst case at ≥ 1 micro, estimate == 0 happens only for a zero-token request or a zero-priced (free) model — so a genuinely free model always 4xxs on reserve.

Location: src/tollgate/application/handlers/reserve.py:145-148; src/tollgate/domain/pricing.py (estimate_micro).

Denying a free model is defensible (there is nothing to gate), but it may surprise callers routing free and paid models through the same guard. Filing to confirm intent: deny, admit-with-zero-hold, or document.

[JumpTechCode]

Fixed in #115 (merged): reserve now admits a zero-cost estimate (a free/zero-priced model or a zero-token request) with a zero-micro hold instead of a 422, so free and paid models transit the same guard; commit reconciles to 0. The now-dead NonPositiveEstimate error was removed.