feat: add stdlib IO and extern function support

- Add stdlib/io.c with PrintLine, Print, PrintInt, PrintFloat, PrintBool, ReadLine
- Support extern function declarations (functions without body)
- Generate C forward declarations for extern functions
- Map Bux type names (String, int, etc.) to C types in codegen
- Update build system to copy all stdlib files
- Hello World example works: prints 'Hello, Bux!'

src/c_backend.nim

@@ -64,7 +64,24 @@ proc typeToC*(typ: Type): string =
     if typ.inner.len > 0:
       return typeToC(typ.inner[0]) & "*"
     return "void*"
-  of tkNamed: return typ.name
+  of tkNamed:
+    # Map common Bux type names to C types
+    case typ.name
+    of "String", "str": return "const char*"
+    of "int": return "int"
+    of "int8": return "int8_t"
+    of "int16": return "int16_t"
+    of "int32": return "int32_t"
+    of "int64": return "int64_t"
+    of "uint": return "unsigned int"
+    of "uint8": return "uint8_t"
+    of "uint16": return "uint16_t"
+    of "uint32": return "uint32_t"
+    of "uint64": return "uint64_t"
+    of "float32": return "float"
+    of "float64": return "double"
+    of "bool": return "bool"
+    else: return typ.name
   of tkTuple: return "void*"  # TODO: proper tuple struct
   of tkFunc: return "void*"  # TODO: function pointer
   else: return "int"
@@ -344,6 +361,16 @@ proc emitEnum*(be: var CBackend, name: string, variants: seq[string]) =
   be.emitLine(&"}} {name};")
   be.emitLine("")
 
+proc emitExternDecl*(be: var CBackend, efunc: HirFunc) =
+  let retType = typeToC(efunc.retType)
+  var params: seq[string] = @[]
+  for p in efunc.params:
+    params.add(&"{typeToC(p.typ)} {p.name}")
+  if params.len == 0:
+    params.add("void")
+  let paramsStr = params.join(", ")
+  be.emitLine(&"extern {retType} {efunc.name}({paramsStr});")
+
 proc emitModule*(be: var CBackend, module: HirModule): string =
   # Header
   be.emitLine("/* Generated by Bux Compiler */")
@@ -360,6 +387,13 @@ proc emitModule*(be: var CBackend, module: HirModule): string =
   if module.structs.len > 0:
     be.emitLine("")
 
+  # Extern function declarations
+  if module.externFuncs.len > 0:
+    be.emitLine("/* Extern function declarations */")
+    for ef in module.externFuncs:
+      be.emitExternDecl(ef)
+    be.emitLine("")
+
   # Struct definitions
   for s in module.structs:
     be.emitStruct(s.name, s.fields)