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, 15 insertions, 0 deletions

diff --git a/tests/unit/stubs/esp_spiffs.h b/tests/unit/stubs/esp_spiffs.h
new file mode 100644
index 0000000..ae6a127
--- /dev/null
+++ b/tests/unit/stubs/esp_spiffs.h
@@ -0,0 +1,15 @@
+#ifndef STUBS_ESP_SPIFFS_H
+#define STUBS_ESP_SPIFFS_H
+
+#include "esp_err.h"
+
+typedef struct {
+    const char *base_path;
+    const char *partition_label;
+    int max_files;
+    bool format_if_mount_failed;
+} esp_vfs_spiffs_conf_t;
+
+static inline esp_err_t esp_vfs_spiffs_register(const esp_vfs_spiffs_conf_t *conf) { (void)conf; return ESP_OK; }
+
+#endif