v0.7.1: rename C core header to victronble_core.h (fixes #2)

src/ held both VictronBLE.h and victronble.h. On case-insensitive
filesystems (Windows, macOS) extracting the library ZIP overwrote one
with the other and the Arduino build failed.

The core header is now victronble_core.h in include/ and src/, and every
include is updated. include/victronble.h stays as a deprecated forwarder
so 0.7.0 Zephyr and host code still builds; it is never in src/.

check_version.sh now fails if two tracked paths differ only in case.

README.md

@@ -488,7 +488,7 @@ The library keeps everything platform-independent except the BLE radio:
 
 ```
 include/
-├── victronble.h            Pure C99 core API — decode one advert, no I/O
+├── victronble_core.h       Pure C99 core API — decode one advert, no I/O
 └── victronble_zephyr.h     Zephyr observer API
 src/
 ├── victronble_core.c       Decrypt + parse; no BLE, no alloc, reentrant