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/freertos/timers.h b/tests/unit/stubs/freertos/timers.h
new file mode 100644
index 0000000..7575807
--- /dev/null
+++ b/tests/unit/stubs/freertos/timers.h
@@ -0,0 +1,15 @@
+#ifndef STUBS_FREERTOS_TIMERS_H
+#define STUBS_FREERTOS_TIMERS_H
+
+#include <stdint.h>
+
+typedef void *TimerHandle_t;
+
+static inline TimerHandle_t xTimerCreate(const char *n, uint32_t pd, int ux, void *id, void *cb) {
+    (void)n; (void)pd; (void)ux; (void)id; (void)cb; return (TimerHandle_t)1;
+}
+static inline int xTimerStart(TimerHandle_t t, uint32_t blk) { (void)t; (void)blk; return 1; }
+static inline int xTimerStop(TimerHandle_t t, uint32_t blk) { (void)t; (void)blk; return 1; }
+static inline void xTimerDelete(TimerHandle_t t, uint32_t blk) { (void)t; (void)blk; }
+
+#endif