fix: drop extern "C" on main to clear -Wpedantic warning

src/main.cpp:161:16: warning: cannot declare '::main' with a linkage
specification [-Wpedantic]

Long-standing upstream OpenInverter code (predates SKUDAK). C++ forbids
linkage specifications on main; with -pedantic in CPPFLAGS this fires
every build.

Confirmed safe to drop:
- libopencm3 vector.c:35 forward-declares int main(void) with default
  linkage and invokes it at boot.
- Linker resolves main by symbol name, not by linkage attribute.
- stm32-sine has the same upstream issue; same fix applies if we ever
  push upstream.

Bump VERSTR -S3 -> -S4 so the OI web UI version field shows the new
build. Verified in the built binary as "4=1.20.R-S4".

Build: clean, no compiler warnings (only the long-standing linker RWX
notice remains, unrelated).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -4,3 +4,4 @@ stm32_charger*
 linker.map
 *.layout
 *.out
+.DS_Store

include/param_prj.h

@@ -126,9 +126,9 @@
 #define CAT_COMM     "Communication"
 
 // SKUDAK customization suffix: bump on every Skudak-side change so the OpenInverter
-// web UI shows a distinct version (e.g. "4=1.20.R-S3") and we can visually confirm
+// web UI shows a distinct version (e.g. "4=1.20.R-S4") and we can visually confirm
 // the right firmware is flashed. Match the stm32-sine -S<N> convention.
-#define VERSTR STRINGIFY(4=VER-S3)
+#define VERSTR STRINGIFY(4=VER-S4)
 
 /***** enums ******/
 

src/main.cpp

@@ -158,7 +158,11 @@ extern "C" void tim2_isr(void)
    scheduler->Run();
 }
 
-extern "C" int main(void)
+// C++ forbids a linkage specification on `main` (-Wpedantic). The reset_handler
+// in libopencm3's vector.c forward-declares `int main(void)` with default
+// linkage and resolves it at link time by symbol name, so dropping `extern "C"`
+// here doesn't affect how main is invoked at boot.
+int main(void)
 {
    extern const TERM_CMD termCmds[];