Add Doxygen, clang-format, cppcheck, and Gitea CI

Doxygen:
- Doxyfile: minimal config, HTML output to docs/, no LaTeX
- @file/@brief on all source files, full @param/@return on public API
- docs/ added to .gitignore

clang-format (14, Linux brace style, 4-space, column 100):
- .clang-format added
- Applied to entire codebase; this commit is the canonical baseline
- just format rewrites in-place; just format-check is the CI gate

cppcheck (--enable=warning,style,performance,portability):
- Linker-symbol pointer comparisons in startup.c suppressed with
  inline cppcheck-suppress (false positives, not real bugs)
- just lint runs cppcheck; zero warnings required to pass

Dockerfile gains clang-format, cppcheck, doxygen packages so all
tools run inside the existing container -- host stays clean.

Gitea Actions (.gitea/workflows/ci.yml):
- Four parallel jobs: build, format, lint, docs
- All jobs use the same Dockerfile-based image
- Doxygen job fails on any warning line in output

include/fhss.h

@@ -1,6 +1,27 @@
+/**
+ * @file fhss.h
+ * @brief FHSS channel sequencer based on AES-128-ECB.
+ *
+ * Both link endpoints derive the same hopping sequence independently from a
+ * shared 128-bit key and a monotonically increasing slot counter.  No
+ * synchronisation traffic is required as long as both sides start from the
+ * same slot.
+ */
 #pragma once
 #include <stdint.h>
 
-void    fhss_init(void);
-uint8_t fhss_next_channel(void);   /* next channel from AES-ECB PRNG sequence */
-void    fhss_sync_tick(void);      /* call every FHSS_DWELL_MS ms */
+/** @brief Reset the slot counter to zero. */
+void fhss_init(void);
+
+/**
+ * @brief Return the next channel in the hopping sequence.
+ *
+ * Encrypts the current slot counter big-endian with AES-128-ECB, returns
+ * @c block[0] % 40, and advances the slot counter.
+ *
+ * @return Channel index in [0, 39].
+ */
+uint8_t fhss_next_channel(void);
+
+/** @brief Advance the slot counter without transmitting (receiver side). */
+void fhss_sync_tick(void);