v0.3.0

Two features, and the second exists because the first made it possible. The
calibration now has a witness for rain, and rain turns out to answer a question
about the probe that no amount of irrigation ever could.

Validation on real hardware is still in progress, as in 0.1.0. The error
budget in docs/design/calibration-method.md is argued from soil physics and
not yet measured against a probe in the ground, and the placement thresholds are
argued rather than measured. Treat this version as field-testable rather than
proven.

There is no 0.2.0 release. That version number was built and installed for field
testing only, and its contents ship here.

Added

• A rain gauge can now be configured, optionally, for the installation. A
  shower that refills the profile counts as an irrigation: it opens a drainage
  window, closes the dry-down cycle and starts the next one, exactly as a valve
  would. Per-event gauges and running totals are both supported, credited by the
  rule that a fall in a counter is a reset and never precipitation, and the
  gauge can be added later to an entry that is already collecting.
• Rain too small to count as a wetting no longer enters the fit unnoticed.
  Readings taken in the rain, or in the drainage window after it, are refused
  under their own name, rain_wetting, instead of being admitted as ordinary
  dry-down points and biasing the slope wet.
• Every cycle now records which water opened it: irrigation, rain, both, or
  unknown. Published on the calibration status entity and in the diagnostics.
• A sixth placement signature, outside_wetted_bulb, which needs the gauge and
  answers a question the other five could not: whether the probe is in a bad
  spot or in a spot the dripper never reaches. Rain wets the whole surface, a
  dripper wets a bulb, and a probe that fills up only when it rains is outside
  that bulb. It needs at least two complete cycles of each kind of water and
  stays silent otherwise.
• A per-probe sheltered from rain option, for a pot under a roof that the
  gauge on the lawn says nothing about.
• A placement diagnostic. The collected cycles are read for five signs that the
  probe is in the wrong place: no response while the reservoir empties, a range
  too coarse to meet the error budget, cycles that disagree with each other, a
  wet anchor that drifts, and an index that jumps while the soil stands still.
  Published on a new probe_placement entity per probe, which carries every
  suspicion with the number behind it, and summarised per probe on the hub.
• A repair for the most severe placement suspicion, one per probe, naming what
  to check and what to do about it, clearing itself when the signature clears.
• The placement diagnostic never blocks a calibration: it sets no status, gates
  nothing and withholds no reading. All but one of its thresholds are argued
  rather than measured, and an unmeasured threshold may advise a user, not
  overrule one. The exception follows from the error budget.
• The rain gauge is treated as a witness that water arrived, never as a
  measurement of how much reached the root zone: heavy rain on dry soil runs
  off, and nothing downstream multiplies by the depth the funnel caught.