Various improvements (#461)

* **New Features**
  * Time and delay utilities added (ticks, ms, µs); SD card now uses an expansion-header CS pin; HTTP downloads warn when run on the GUI task and yield to avoid blocking.

* **Bug Fixes / Reliability**
  * Many hard-crash paths converted to guarded checks to reduce abrupt termination and improve stability.

* **Tests**
  * Unit tests added to validate time and delay accuracy.

* **Chores**
  * License header and build/macro updates.

CODING_STYLE.md

@@ -46,14 +46,6 @@ void getLimit() {
 }
 ```
 
-### Preprocessor
-
-Preprocessor functions are written in snake-case and prefixed with `tt_`, for example:
-
-```c++
-#define tt_check(x) if (!(x)) { /* .. */ }
-```
-
 ### Type names
 
 Consts are lower camel case with capital letters.