fix(firmware): fall detection false positives + 4MB flash support (#263, #265)

Issue #263: Default fall_thresh raised from 2.0 to 15.0 rad/s² — normal
walking produces accelerations of 2.5-5.0 which triggered constant false
"Fall Detected" alerts. Added consecutive-frame requirement (3 frames)
and 5-second cooldown debounce to prevent alert storms.

Issue #265: Added partitions_4mb.csv and sdkconfig.defaults.4mb for
ESP32-S3 boards with 4MB flash (e.g. SuperMini). OTA slots are 1.856MB
each, fitting the ~978KB firmware binary with room to spare.

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

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

@@ -42,6 +42,10 @@
 #define EDGE_CALIB_FRAMES     1200  /**< Frames for adaptive calibration (~60s at 20 Hz). */
 #define EDGE_CALIB_SIGMA_MULT 3.0f  /**< Threshold = mean + 3*sigma of ambient. */
 
+/* ---- Fall detection ---- */
+#define EDGE_FALL_COOLDOWN_MS 5000  /**< Minimum ms between fall alerts (debounce). */
+#define EDGE_FALL_CONSEC_MIN  3     /**< Consecutive frames above threshold to trigger. */
+
 /* ---- SPSC ring buffer slot ---- */
 typedef struct {
     uint8_t  iq_data[EDGE_MAX_IQ_BYTES]; /**< Raw I/Q bytes from CSI callback. */