E4-2: write explicit prototypes for the 13 functions that rely on Arduino-generated ones #261

Epic: #212

Mechanical, declaration-only. These are the only 13 authored functions called before their definition with no explicit forward declaration, which is the exact set that breaks the moment their code leaves the .ino (Appendix F). The regex-era plan said 70; the linker says 13.

calcBearing  calcDistanceKm  calculateForecast  getCardinal  getIaqQualityText
getTrendArrow  isBatteryConnected  loadBsecFromFRAM  loadBsecState  mktimeUTC
readBME688  readIMU  readSHT41

Add each prototype in the existing forward-declaration block. Four of the thirteen (calcBearing, calcDistanceKm, getCardinal, getIaqQualityText) are also leaves and become movable the moment this lands.

Independent of E4-1; either can land first.

Acceptance

• python scripts/reference-graph.py reports 0 functions relying on generated prototypes
• Build succeeds; nm symbol set, sizes and types unchanged (prototypes emit nothing)

Evidence: docs/god-object-inventory.md. Regenerate the figures with python scripts/reference-graph.py --appendix.