Add TDD calculation for Loop users and Basal Variability chart

[MtlPhil]

Feat: TDD calculation for Loop and Basal Variability chart

Summary

Two additions to the Statistics view for Loop users, plus a set of correctness fixes for the underlying TDD data pipeline.

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1. TDD (Total Daily Dose) calculation for Loop

Displays the rolling 24-hour total daily dose in the info table for Loop users. TDD is computed from the treatments endpoint only (not device status) to avoid double-counting, then cached to Storage so it is available across views.

Data pipeline

The calculation lives in DeviceStatusLoop and runs whenever treatments or device status are refreshed:

• Basal: reconstructed from the scheduled basal profile, filling gaps between actual temp-basal records. A guard prevents gap-fill before the first treatment entry, avoiding spurious additions at the tail.
• Bolus: summed directly from treatments (normal + SMB).
• Race condition guard: Treatments.swift now posts a .treatmentsUpdated notification that DeviceStatusLoop observes, ensuring TDD is not computed until both datasets are available.

Fixes included

Fix	Description
Blank on first load	Wait for .treatmentsUpdated before first calculation
Basal gap before first entry	Guard the retroactive gap-fill against the first treatment timestamp
Crash on empty basalData	Nil-check basalData.first before accessing it

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2. Basal Variability chart

A new chart in the Statistics view that plots scheduled vs. actual basal delivery over the selected date range, making it easy to see how often and by how much Loop is adjusting basal.

New files

File	Purpose
LoopFollow/Stats/BasalVariability/BasalVariabilityCalculator.swift	Aggregates scheduled and actual basal by hour
LoopFollow/Stats/BasalVariability/BasalVariabilityDataPoint.swift	Data model
LoopFollow/Stats/BasalVariability/BasalVariabilityGraphView.swift	SwiftUI chart view
LoopFollow/Stats/BasalVariability/BasalVariabilityView.swift	Container view with legend
LoopFollow/Stats/BasalVariability/BasalVariabilityViewModel.swift	Drives the chart from AggregatedStatsViewModel

The chart is gated behind Loop being the active closed-loop system and is inserted into AggregatedStatsView between the TIR section and the existing tables.

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Files changed

File	Change
LoopFollow/Controllers/Nightscout/DeviceStatus.swift	Route TDD reads through treatments only
LoopFollow/Controllers/Nightscout/DeviceStatusLoop.swift	Full TDD calculation with basal reconstruction
LoopFollow/Controllers/Nightscout/Treatments.swift	Post .treatmentsUpdated notification
LoopFollow/Stats/AggregatedStatsView.swift	Wire Basal Variability chart into Statistics
LoopFollow/Stats/AggregatedStatsViewModel.swift	Expose BasalVariabilityViewModel
LoopFollow/Stats/BasalVariability/	New — five files (see above)

[MtlPhil]

Corrected lint errors.