feat: double-buffer write pattern for local storage

[dougborg]

Summary

Implement a ping-pong double-buffer to decouple scan processing from flash/SD writes. This prevents storage write latency from blocking the scan pipeline.

Inspired by the ESP32DualBandWardriver's Buffer class, which alternates two buffers for this exact purpose.

Design

• Two fixed-size buffers alternate: one accumulates new data, one flushes to storage
• When the active buffer is full (or a time threshold is hit), swap and signal the flush task
• Flush task writes the completed buffer to flash/SD while new data goes to the other buffer
• Generic PingPongBuffer<const N: usize> in the library crate — reusable across storage backends

Implementation

• Fits naturally into Embassy async — flush task awaits on a swap signal (Signal or Channel)
• Buffer size tunable per platform:
  • ESP32 (M5StickC): ~2KB per buffer (tight DRAM)
  • ESP32-S3 (XIAO): larger, especially with PSRAM
• no_std + alloc compatible (or purely static with const generics)

Use Cases

• Match log (NDJSON lines appended to flash/SD)
• WiGLE CSV append (lib: WiGLE CSV v1.6 export module (export.rs) #31)
• Observation buffer for local storage (feat: ESP-IDF sensor storage — observation buffer, match log, and watchlist #60)
• Any future export format (arch: platform-agnostic storage traits and core data types #54)

Relates To

• feat: ESP-IDF sensor storage — observation buffer, match log, and watchlist #60 — ESP-IDF sensor storage (primary consumer)
• lib: WiGLE CSV v1.6 export module (export.rs) #31 — WiGLE export module
• arch: platform-agnostic storage traits and core data types #54 — export/storage planning