feat: UI for pairing Whisperer sensors to zones; prefer sensor over daily forecast

[simons-plugins]

Problem

Today a user observed their Pixie zone showing 89% moisture in Indigo while the Netro mobile app showed 24% for the same zone. Investigation confirmed:

• /moistures.json returns Netro's smart-model daily prediction (model assumes saturation post-irrigation and decays over days) — for this zone today, 89%.
• The Whisperer sensor paired to that zone shows the actual soil reading, ~22–31% throughout the day, currently 24%.
• Netro's mobile app renders the Whisperer reading on the zone tile when a Whisperer is paired in the account. The public API doesn't expose this pairing.

Confirmed by comparing yesterday's data on the same zone:

• Whisperer sensor range for 2026-04-21: 19–29% (24 readings, hourly)
• /moistures.json row for same zone, same date: 89%

So /moistures.json is not the sensor reading — it's a model prediction that can be dramatically different from ground truth. Plugin reads both correctly, but they surface on different Indigo devices with no linkage.

Proposed solution

Add a per-zone Whisperer pairing in device config UI (Devices.xml + ConfigUI callback):

1. On the zone device config, add a dropdown linkedWhispererDeviceId populated with this plugin's Whisperer devices + an "(unpaired)" option.
2. In _update_zone_devices / process_zone_moisture, when a Whisperer is paired: write the Whisperer's latest soilMoisture / sensorValue into the zone's moisture state instead of (or in addition to) the daily forecast row.
3. Add a separate state moistureForecast on the zone so the daily /moistures.json value is still observable (useful for comparing model vs reality, tuning schedules, etc.).
4. No auto-pairing — explicit config only, to avoid brittle name-matching.

Out of scope

• Not part of PR fix: guard against stale EXECUTING schedules from Netro cloud #53 (stuck EXECUTING fix). This is a separate feature.
• Does not require Netro API changes — pairing lives entirely in the plugin.

Documentation follow-up

docs/API_NOTES.md §6 currently says moisture "can be 12–24 hours old" — update to clarify it's a model prediction, not a sampled sensor value, and can diverge significantly from a paired Whisperer's actual reading.

Acceptance

• Zone device config has a Whisperer dropdown
• When paired, zone moisture state mirrors Whisperer's current reading
• moistureForecast state exposes the /moistures.json daily value
• Tests covering paired / unpaired / Whisperer unavailable paths
• API_NOTES.md updated to describe /moistures.json as predicted, not observed