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, 25 insertions, 0 deletions

diff --git a/tests/unit/stubs/esp_tls.h b/tests/unit/stubs/esp_tls.h
new file mode 100644
index 0000000..7ded63a
--- /dev/null
+++ b/tests/unit/stubs/esp_tls.h
@@ -0,0 +1,25 @@
+#ifndef STUBS_ESP_TLS_H
+#define STUBS_ESP_TLS_H
+
+#include "esp_err.h"
+
+typedef struct esp_tls esp_tls_t;
+
+typedef struct {
+    void *crt_bundle_attach;
+    int use_global_ca_store;
+} esp_tls_cfg_t;
+
+static inline esp_tls_t *esp_tls_init(void) { return (esp_tls_t*)1; }
+static inline int esp_tls_conn_new_sync(const char *h, int hl, int port, const esp_tls_cfg_t *cfg, esp_tls_t *tls) {
+    (void)h; (void)hl; (void)port; (void)cfg; (void)tls; return -1;
+}
+static inline int esp_tls_conn_write(esp_tls_t *tls, const void *data, size_t len) {
+    (void)tls; (void)data; (void)len; return len;
+}
+static inline int esp_tls_conn_read(esp_tls_t *tls, void *data, size_t len) {
+    (void)tls; (void)data; (void)len; return 0;
+}
+static inline void esp_tls_conn_destroy(esp_tls_t *tls) { (void)tls; }
+
+#endif