[gwosd]

The interpolation part of #545 on its own, without any design changes, so it can be reviewed separately. The weight curve now comes from state_space, a smoother of the log weight with a fading trend, week-scale changes, water and the scale error, instead of the Gaussian kernel. It runs in linear time and learns how much each diary scatters.

Changes

• Curve and slope from state_space; the strengths become slope sds scaled to body weight, "none" takes its rate from medium
• The day-to-day and week-to-week scatter and the scale error are refitted from the fifth day with readings
• The cards read the curve's change over 14 days, the forecasts average it with the trend's fading rate; the target threshold is relative to body weight
• The interpolation cache is removed, since the splash recomputed on every start anyway; the released key is deleted once
• Fixed: the grid lost its last day when that was read earlier in the day than the first, and with "none" the rate was read from a week earlier
• state_space comes from its unreleased priors branch (feat: hold parameters, fit under priors, sum chosen components state_space#19); 0.3.0 must be released before v2 ships

The scientific chart and its band, the dashed projection, the growing profile and sharing the summary follow in later PRs.

Closes #481
Closes #279