Phase 7.10: command-line args + buxc2 check/build working

Runtime:
- bux_argc()/bux_argv() — command-line arg access from Bux programs
- g_argc/g_argv globals — set by C main() wrapper

Compiler:
- C backend generates extern g_argc/g_argv + int main(argc,argv) wrapper
- main.bux reads args via bux_argc/bux_argv and passes to Cli_Run

buxc2 verified:
- buxc2 version — reads CLI args correctly
- buxc2 check <file.bux> — lexes, parses, type-checks, generates C
- buxc2 build <file.bux> <output.c> — writes C output

PLAN.md updated with Phase 7.10 status and remaining work.
All 18 examples pass, all unit tests pass.

src/c_backend.nim

@@ -544,7 +544,11 @@ proc emitModule*(be: var CBackend, module: HirModule): string =
   # Generate C main wrapper if Bux Main exists
   if hasMain:
     be.emitLine("/* C entry point wrapper */")
+    be.emitLine("extern int g_argc;")
+    be.emitLine("extern char** g_argv;")
     be.emitLine("int main(int argc, char** argv) {")
+    be.emitLine("    g_argc = argc;")
+    be.emitLine("    g_argv = argv;")
     be.emitLine("    return Main();")
     be.emitLine("}")
     be.emitLine("")