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, 18 insertions, 0 deletions

diff --git a/tests/unit/stubs/esp_err.h b/tests/unit/stubs/esp_err.h
new file mode 100644
index 0000000..84c3734
--- /dev/null
+++ b/tests/unit/stubs/esp_err.h
@@ -0,0 +1,18 @@
+#ifndef STUBS_ESP_ERR_H
+#define STUBS_ESP_ERR_H
+
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef int esp_err_t;
+
+#define ESP_OK          0
+#define ESP_FAIL        -1
+#define ESP_ERR_INVALID_ARG  0x102
+#define ESP_ERR_NO_MEM       0x101
+#define ESP_ERR_NOT_FOUND    0x104
+
+#define ESP_ERROR_CHECK(x) do { if ((x) != 0) { fprintf(stderr, "ESP_ERROR_CHECK failed: 0x%x\n", (int)(x)); abort(); } } while(0)
+
+#endif