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.

examples/NativeDecode/README.md

@@ -79,6 +79,6 @@ victronble_device_type_str()
 victronble_state_str()
 ```
 
-That is the whole portable core. See `include/victronble.h` for the record
+That is the whole portable core. See `include/victronble_core.h` for the record
 structures, and `samples/observer/` for the same printing logic driven by a
 live BLE scan under Zephyr.