classifyValarms: check sign of dur.toSeconds() before preset lookup.
Positive value = alarm fires after event (RFC 5545 TRIGGER:+PT15M or
TRIGGER;RELATED=END:PTNm) → return {kind:'custom'} for preserve path.
Compute leadMinutes as -seconds/60 (was Math.abs) for negative triggers.
Prevents alarm direction inversion: +PT15M was being stored as 15-min-before
lead and re-fired at dtstartUtc-15min — the opposite of the original intent.