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/sdkconfig.defaults.4mb

@@ -0,0 +1,29 @@
+# ESP32-S3 CSI Node — 4MB Flash SDK Configuration (issue #265)
+# For boards with 4MB flash (e.g. ESP32-S3 SuperMini 4MB).
+#
+# Build: cp sdkconfig.defaults.4mb sdkconfig.defaults && idf.py set-target esp32s3 && idf.py build
+# Or:    idf.py -D SDKCONFIG_DEFAULTS="sdkconfig.defaults.4mb" set-target esp32s3 && idf.py build
+
+CONFIG_IDF_TARGET="esp32s3"
+
+# 4MB flash partition table
+CONFIG_PARTITION_TABLE_CUSTOM=y
+CONFIG_PARTITION_TABLE_CUSTOM_FILENAME="partitions_4mb.csv"
+CONFIG_ESPTOOLPY_FLASHSIZE_4MB=y
+CONFIG_ESPTOOLPY_FLASHSIZE="4MB"
+
+# Compiler: optimize for size (critical for 4MB)
+CONFIG_COMPILER_OPTIMIZATION_SIZE=y
+
+# CSI support
+CONFIG_ESP_WIFI_CSI_ENABLED=y
+
+# Disable display support to save flash (ADR-045 display requires 8MB)
+# CONFIG_DISPLAY_ENABLE is not set
+
+# Reduce logging to save flash
+CONFIG_BOOTLOADER_LOG_LEVEL_WARN=y
+CONFIG_LOG_DEFAULT_LEVEL_INFO=y
+
+CONFIG_LWIP_SO_RCVBUF=y
+CONFIG_ESP_MAIN_TASK_STACK_SIZE=8192