B7: Epic B integration test — PlatformIO build on hardware #161

_{Recreated from Strycher/Field_Compass-archive#161 — originally opened by @Strycher on 2026-08-09. Credentials redacted where present.}

Epic: #154

Epic B integration test. Human sign-off required.

Test

Flash the PlatformIO artifact and verify every subsystem: display, touch, GPS fix, IMU heading, SD write, FRAM buffer, SHT41/BME688 reads, battery gauge, web endpoints.

Acceptance criteria

• Flash and RAM figures within a documented tolerance of the arduino-cli baseline (B6 parity check)
• No regression in boot time or UI responsiveness
• Human confirms "verified on device"
• Only after this passes is arduino-cli retired and the fallback path closed

Blocks Epic C (#155).