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.

stdlib/runtime.c

@@ -7,6 +7,16 @@
 #include <stdbool.h>
 #include <string.h>
 
+/* Command-line argument storage */
+int g_argc = 0;
+char** g_argv = NULL;
+
+int bux_argc(void) { return g_argc; }
+char* bux_argv(int index) {
+    if (index < 0 || index >= g_argc) return "";
+    return g_argv[index];
+}
+
 /* Memory allocation */
 void* bux_alloc(size_t size) {
     void* ptr = malloc(size);