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, 40 insertions, 0 deletions

diff --git a/tests/unit/stubs/esp_wifi.h b/tests/unit/stubs/esp_wifi.h
new file mode 100644
index 0000000..6aa5787
--- /dev/null
+++ b/tests/unit/stubs/esp_wifi.h
@@ -0,0 +1,40 @@
+#ifndef STUBS_ESP_WIFI_H
+#define STUBS_ESP_WIFI_H
+
+#include <stdint.h>
+#include <string.h>
+#include "esp_err.h"
+
+#define WIFI_IF_STA 0
+#define WIFI_IF_AP  1
+
+#define WIFI_AUTH_WPA2_PSK 3
+#define WIFI_AUTH_OPEN     0
+
+#define WIFI_MODE_APSTA 3
+
+typedef struct {
+    struct {
+        uint8_t ssid[32];
+        uint8_t password[64];
+        uint8_t channel;
+        uint8_t max_connection;
+        uint8_t ssid_hidden;
+        int authmode;
+    } ap;
+    struct {
+        uint8_t ssid[32];
+        uint8_t password[64];
+        int threshold;
+        struct {
+            int authmode;
+        } sta;
+    } sta;
+} wifi_config_t;
+
+static inline esp_err_t esp_wifi_set_mac(int ifx, const uint8_t *mac) { (void)ifx; (void)mac; return ESP_OK; }
+static inline esp_err_t esp_wifi_set_config(int ifx, const wifi_config_t *cfg) { (void)ifx; (void)cfg; return ESP_OK; }
+static inline esp_err_t esp_wifi_set_mode(uint8_t mode) { (void)mode; return ESP_OK; }
+static inline esp_err_t esp_wifi_start(void) { return ESP_OK; }
+
+#endif