fix(firmware): enable CSI in sdkconfig and add build guard (ADR-057)

The committed sdkconfig had CONFIG_ESP_WIFI_CSI_ENABLED disabled, causing
all builds to crash at runtime with "CSI not enabled in menuconfig".
Root cause: sdkconfig.defaults.template existed but ESP-IDF only reads
sdkconfig.defaults (no .template suffix).

Fixes:
- Add sdkconfig.defaults with CONFIG_ESP_WIFI_CSI_ENABLED=y
- Add #error compile guard in csi_collector.c to prevent recurrence
- Fix NVS encryption default (requires eFuse, breaks clean builds)

Verified: Docker build + flash to ESP32-S3 + CSI callbacks confirmed.

Closes #241
Relates to #223, #238, #234, #210, #190

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

firmware/esp32-csi-node/sdkconfig.defaults

@@ -0,0 +1,33 @@
+# ESP32-S3 CSI Node — Default SDK Configuration
+# This file is applied automatically by idf.py when no sdkconfig exists.
+
+# Target: ESP32-S3
+CONFIG_IDF_TARGET="esp32s3"
+
+# Use custom partition table (8MB flash with OTA — ADR-045)
+CONFIG_PARTITION_TABLE_CUSTOM=y
+CONFIG_PARTITION_TABLE_CUSTOM_FILENAME="partitions_display.csv"
+
+# Flash configuration: 8MB (Quad SPI)
+CONFIG_ESPTOOLPY_FLASHSIZE_8MB=y
+CONFIG_ESPTOOLPY_FLASHSIZE="8MB"
+
+# Compiler optimization: optimize for size to reduce binary
+CONFIG_COMPILER_OPTIMIZATION_SIZE=y
+
+# Enable CSI (Channel State Information) in WiFi driver
+CONFIG_ESP_WIFI_CSI_ENABLED=y
+
+# NVS encryption disabled by default (requires eFuse provisioning).
+# Enable only after burning HMAC key to eFuse block.
+# CONFIG_NVS_ENCRYPTION is not set
+
+# Disable unused features to reduce binary size
+CONFIG_BOOTLOADER_LOG_LEVEL_WARN=y
+CONFIG_LOG_DEFAULT_LEVEL_INFO=y
+
+# LWIP: enable extended socket options for UDP multicast
+CONFIG_LWIP_SO_RCVBUF=y
+
+# FreeRTOS: increase task stack for CSI processing
+CONFIG_ESP_MAIN_TASK_STACK_SIZE=8192