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.

samples/observer/src/main.c

@@ -62,7 +62,7 @@ static const struct {
 
 /*
  * A float field the device did not send comes back as NAN (see
- * include/victronble.h). Render that as "n/a" rather than letting "nan" leak
+ * include/victronble_core.h). Render that as "n/a" rather than letting "nan" leak
  * into the log — a missing load-current reading is not a fault. The unit goes
  * in here too, so an absent field reads "n/a" and not "n/a A".
  *