Testing infrastructure: AGENTS.md rules + unit test framework + geohash tests (11/11 pass)

- Add AGENTS.md: full project context + mandatory testing rules for AI sessions
- Add tests/unit/ with host-compiled C unit test infrastructure
- Clean stubs approach: ESP-IDF type stubs in tests/unit/stubs/, no source modifications
- Fix geohash.c bit extraction bug (3-byte span) found by unit tests
- test_geohash: 11/11 passing with reference vectors (Munich, NYC, origin, boundaries)

1 files changed, 12 insertions, 0 deletions

diff --git a/tests/unit/stubs/nvs_flash.h b/tests/unit/stubs/nvs_flash.h
new file mode 100644
index 0000000..4424a9a
--- /dev/null
+++ b/tests/unit/stubs/nvs_flash.h
@@ -0,0 +1,12 @@
+#ifndef STUBS_NVS_FLASH_H
+#define STUBS_NVS_FLASH_H
+
+#include "esp_err.h"
+
+#define ESP_ERR_NVS_NO_FREE_PAGES    0x1101
+#define ESP_ERR_NVS_NEW_VERSION_FOUND 0x1102
+
+static inline esp_err_t nvs_flash_init(void) { return ESP_OK; }
+static inline esp_err_t nvs_flash_erase(void) { return ESP_OK; }
+
+#endif