fix(blitter): let each SIMD .c select its own arch when built standalone

CI's 'Run SIMD blitter tests' step compiles test_blitter_simd.c plus
one blitter_simd_<arch>.c by hand, without the Makefile's
-DBLITTER_SIMD_<ARCH>. blitter_simd.h then fell through to the scalar
header while the .c also included its own, defining both inline sets
and colliding -- which is why every platform failed the same step
while the core build stayed green.

Each .c now defines its own macro before including blitter_simd.h, so
it is self-contained however it is compiled. The Makefile's -D still
applies, and a genuine -D/.c mismatch is still a hard error.

Verified with CI's exact command: standalone neon and scalar both
compile and report 56103/56103.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/tom/blitter_simd_neon.c

@@ -7,6 +7,16 @@
  * uses to exercise the very same code the core runs.
  */
 
+/* Select our own arch before blitter_simd.h picks one.  This file IS
+ * the neon implementation, so it must get the neon inline set even when
+ * compiled standalone without the Makefile's -DBLITTER_SIMD_NEON --
+ * as .github/workflows/c-cpp.yml does when it builds
+ * test_blitter_simd by hand.  Without this the header chain pulled in
+ * the scalar set as well and the two collided. */
+#ifndef BLITTER_SIMD_NEON
+#define BLITTER_SIMD_NEON 1
+#endif
+
 #include "blitter_simd.h"
 #include "blitter_simd_neon.h"
 

src/tom/blitter_simd_scalar.c

@@ -7,6 +7,16 @@
  * uses to exercise the very same code the core runs.
  */
 
+/* Select our own arch before blitter_simd.h picks one.  This file IS
+ * the scalar implementation, so it must get the scalar inline set even when
+ * compiled standalone without the Makefile's -DBLITTER_SIMD_SCALAR --
+ * as .github/workflows/c-cpp.yml does when it builds
+ * test_blitter_simd by hand.  Without this the header chain pulled in
+ * the scalar set as well and the two collided. */
+#ifndef BLITTER_SIMD_SCALAR
+#define BLITTER_SIMD_SCALAR 1
+#endif
+
 #include "blitter_simd.h"
 #include "blitter_simd_scalar.h"
 

src/tom/blitter_simd_sse2.c

@@ -7,6 +7,16 @@
  * uses to exercise the very same code the core runs.
  */
 
+/* Select our own arch before blitter_simd.h picks one.  This file IS
+ * the sse2 implementation, so it must get the sse2 inline set even when
+ * compiled standalone without the Makefile's -DBLITTER_SIMD_SSE2 --
+ * as .github/workflows/c-cpp.yml does when it builds
+ * test_blitter_simd by hand.  Without this the header chain pulled in
+ * the scalar set as well and the two collided. */
+#ifndef BLITTER_SIMD_SSE2
+#define BLITTER_SIMD_SSE2 1
+#endif
+
 #include "blitter_simd.h"
 #include "blitter_simd_sse2.h"