Covers the tax-sensitive bits the dunning rework added (previously untested):
flat vs percent fee, the VAT toggle applying the org rate AND no-op'ing when
the org has no VAT rate, per-reminder accumulation (2nd=1x / 3rd=2x), the
invoice total staying immutable while the fee is tracked, and the 3-reminder
cap. Exports the fee resolvers + applyReminder for testing; PDF render stubbed
(flaky in CI, verified manually). 6/6 pass.