fix(firmware): CSI not enabled in sdkconfig — build guard + sdkconfig.defaults (ADR-057)

[ruvnet]

Summary

The committed sdkconfig had CONFIG_ESP_WIFI_CSI_ENABLED disabled (line 1135: # CONFIG_ESP_WIFI_CSI_ENABLED is not set), causing a runtime error on all builds:

E (6700) wifi:CSI not enabled in menuconfig!

Root cause: sdkconfig.defaults.template had the correct setting, but ESP-IDF only reads sdkconfig.defaults (no .template suffix). No sdkconfig.defaults file was committed.

Fixes Applied (ADR-057)

1. sdkconfig line 1135: Changed to CONFIG_ESP_WIFI_CSI_ENABLED=y
2. sdkconfig.defaults: Created from template so fresh builds inherit correct defaults
3. csi_collector.c: Added #error compile-time guard — turns confusing runtime crash into clear build error with fix instructions

Related Issues

• What exact ESP32-S3 is supported? #223 — User reports "CSI not enabled in menuconfig" with pre-built firmware
• 0.4 firmware not working #238 — 0.4 firmware not working (original ESP32, unsupported)
• Support for ESP32 C3? #234 — ESP32-C3 support request (unsupported, RISC-V)
• unable to flash my ESP32-S3 Super Mini using the MSI program #210 — Flashing issues (tooling, not firmware code)
• Unable to receive data from eps32S3 #190 — No data received (user config, not firmware bug)

Hardware Support Matrix

Variant	Supported	Notes
ESP32-S3	Yes	All variants (N4R2, N8R8, N16R8, Super Mini, DevKitC)
ESP32 (original)	No	Partial CSI, different architecture
ESP32-C3	No	Has CSI but needs separate RISC-V build target
ESP32-C6	No	Has CSI but needs separate RISC-V build target

Test Plan

• Rebuild firmware with Docker: verify CONFIG_ESP_WIFI_CSI_ENABLED=y in build output
• Flash ESP32-S3 and confirm CSI callbacks fire (no runtime error)
• Delete sdkconfig, run idf.py build — verify sdkconfig.defaults applies CSI setting
• Comment out CONFIG_ESP_WIFI_CSI_ENABLED=y in sdkconfig — verify compile fails with #error