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, 17 insertions, 0 deletions

diff --git a/tests/unit/stubs/esp_netif.h b/tests/unit/stubs/esp_netif.h
new file mode 100644
index 0000000..f009537
--- /dev/null
+++ b/tests/unit/stubs/esp_netif.h
@@ -0,0 +1,17 @@
+#ifndef STUBS_ESP_NETIF_H
+#define STUBS_ESP_NETIF_H
+
+#include <stdint.h>
+
+typedef struct {
+    uint32_t addr;
+} esp_ip4_addr_t;
+
+#define IPSTR "%d.%d.%d.%d"
+#define IP2STR(ip) ((ip)->addr & 0xff), (((ip)->addr >> 8) & 0xff), (((ip)->addr >> 16) & 0xff), (((ip)->addr >> 24) & 0xff)
+
+static inline void IP4_ADDR(esp_ip4_addr_t *ip, uint8_t a, uint8_t b, uint8_t c, uint8_t d) {
+    ip->addr = ((uint32_t)a) | ((uint32_t)b << 8) | ((uint32_t)c << 16) | ((uint32_t)d << 24);
+}
+
+#endif