fix(firmware): stack overflow risk + tick-rate independence (review findings)

Critical fixes from deep review:

1. **Stack overflow prevention**: Moved BPM scratch buffers (br_buf, hr_buf)
   from stack to static storage in both process_frame() and
   update_multi_person_vitals(). Combined stack was ~6.5-7.5 KB of 8 KB
   limit — now reduced by ~4 KB to safe margins.

2. **Tick-rate independence**: Post-batch yield now uses
   pdMS_TO_TICKS(20) with min-1 guard instead of raw vTaskDelay(2).
   Previously assumed 100Hz tick rate.

3. **EDGE_BATCH_LIMIT to header**: Moved from local const to
   edge_processing.h #define for configurability.

Firmware builds clean at 843 KB.

Co-Authored-By: claude-flow <ruv@ruv.net>

firmware/esp32-csi-node/main/edge_processing.h

@@ -46,6 +46,9 @@
 #define EDGE_FALL_COOLDOWN_MS 5000  /**< Minimum ms between fall alerts (debounce). */
 #define EDGE_FALL_CONSEC_MIN  3     /**< Consecutive frames above threshold to trigger. */
 
+/* ---- DSP task tuning ---- */
+#define EDGE_BATCH_LIMIT      4     /**< Max frames per batch before longer yield. */
+
 /* ---- SPSC ring buffer slot ---- */
 typedef struct {
     uint8_t  iq_data[EDGE_MAX_IQ_BYTES]; /**< Raw I/Q bytes from CSI callback. */