feat(firmware): QEMU ESP32-S3 testing platform (ADR-061)

Implement full QEMU emulation framework for firmware testing without
physical hardware:

Mock CSI Generator (mock_csi.c):
- 10 test scenarios: empty room, static/walking person, fall, multi-person,
  channel sweep, MAC filter, ring overflow, boundary RSSI, zero-length
- Physics-based signal model with breathing modulation and Doppler
- LFSR pseudo-random noise, CONFIG_CSI_MOCK_ENABLED Kconfig guard
- Scenario 255 runs all sequentially

QEMU Runner & CI:
- qemu-esp32s3-test.sh: build, merge flash image, run QEMU, validate
- validate_qemu_output.py: 14 automated checks (boot, NVS, edge, vitals,
  crash detection) with colored output and severity-based exit codes
- generate_nvs_matrix.py: 14 NVS provisioning configs for matrix testing
- firmware-qemu.yml: GitHub Actions CI with 4-scenario matrix

Fuzz Testing:
- 3 libFuzzer targets: CSI serialize, NVS config validation, ring buffer
- Host-compilable ESP-IDF stubs (no ESP-IDF dependency for fuzzing)
- 6 seed corpus files for guided fuzzing
- Makefile with ASAN + UBSAN sanitizers

Documentation:
- firmware/esp32-csi-node/README.md: comprehensive QEMU testing guide
- Root README.md: collapsed QEMU testing section

Build verified: normal firmware build (RC=0) with mock_csi excluded.

Closes #259

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

firmware/esp32-csi-node/main/CMakeLists.txt

@@ -6,6 +6,11 @@ set(SRCS
 
 set(REQUIRES "")
 
+# ADR-061: Mock CSI generator for QEMU testing
+if(CONFIG_CSI_MOCK_ENABLED)
+    list(APPEND SRCS "mock_csi.c")
+endif()
+
 # ADR-045: AMOLED display support (compile-time optional)
 if(CONFIG_DISPLAY_ENABLE)
     list(APPEND SRCS "display_hal.c" "display_ui.c" "display_task.c")