Reference-tide fits + derived-slack gates: CHS constituents 0.2.0

fitTideStation: fetch a reference tide port's wlp, decimate to 15 min, fit the standard harmonic basis → a tide-harmonic record (no flood/ebb axis).
Emit derived-slack gate records from the station registry's derived block (derivedGates, derivedSlackRecord): reference port + HW/LW lags, no constituents. buildBundle fits each derived gate's reference port once and appends both records.
Point Atkinson fits to RMS 0.075 m; HW/LW timing within ~6 min of CHS out-of-sample.
FittedStation.type widened to "harmonic" | "tide-harmonic"; floodDirection/ebbDirection now optional (absent on a tide fit).
chs-derived output stays gitignored — not for redistribution.