Three failing tests pinning Task 4.3's mode-toggle state hygiene:
- clears threshold+windowSeconds on COUNT_IN_WINDOW -> PER_EXCHANGE
- returns to defaults (not stale values) on PER_EXCHANGE -> COUNT_IN_WINDOW
- forces reNotifyMinutes=0 and forDurationSeconds=0 on PER_EXCHANGE
Targets a to-be-introduced pure helper `applyFireModeChange(form, newMode)`
in form-state.ts. Task 4.3 will implement the helper and wire it into
ExchangeMatchForm so the Fire-mode <Select> calls it instead of the current
raw patch({ fireMode }) that leaves stale fields.
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