fix: state libretro core versions from their code

Each libretro profile records version_basis and core_version is read
there at its pin. snes9x states 1.63 in snes9x.h, nestopia 2.0.0, mgba
0.11.0, vice 3.10, atari800's libretro tree 7.0.0. Cores whose build
takes its version from git carry the nearest tag in describe form
(flycast v2.7-90-ge88b0ce, ppsspp, duckstation, gearcoleco), placeholders
copied from .info files (Git, SVN) are replaced by the code's statement,
and a code that states none is null with kind none.

emulators/blastem.yml

@@ -7,6 +7,10 @@ profiled_date: "2026-10-20"
 source_commit: "ab743653ed110eeb7b1fe64d00cc5f9279c2bfec"
 prose_written_at: "1e0de94dc7e669c0925a22c0fccf6cdc837af0a0"
 core_version: "1.0.1-pre"
+version_basis:
+  kind: file
+  path: "version.inc"
+  pattern: '#define\s+BLASTEM_VERSION\s+"([^"]+)'
 display_name: "Sega - Mega Drive - Genesis (BlastEm)"
 cores:
   - blastem