Firmware: log capture sessions to the V4 onboard microSD

[turfptax]

FlexGrid V4 hardware includes an onboard microSD slot (added in commit 604f854) so the band can capture sensor sessions standalone, without a tethered PC.

This issue tracks the firmware side:

• SPI driver + FAT mount for the microSD on the V4 pinout
• Write capture sessions to card (same packet/session format the PC pipeline ingests, see OpenMuscle-Software docs/protocol.md)
• Session file naming and rotation (timestamped, safe on power loss)
• A way to mark/label sessions without a PC (button or menu on the OLED)
• Import path: PC tooling reads card sessions into the normal data pipeline

Firmware develops in OpenMuscle-Software/embedded and will promote to FlexGridV4-Firmware when V4 hardware stabilizes, per the repo layout doc.

Hardware capability suggested by @HeatRIderHR in Open-Muscle/OpenMuscle-Hub#2.