feat: add Std::Fmt — string formatting module

- C runtime: bux_float_to_string() for float64 → String conversion
- String.bux: add String_FromFloat, extern decl for bux_float_to_string
- library/std/Fmt.bux: Fmt_Fmt1, Fmt_Fmt2, Fmt_Fmt3, Fmt_FmtInt, Fmt_FmtInt2,
  Fmt_FmtFloat, Fmt_FmtBool — positional {0}, {1}, {2} placeholders
- Refactor examples to use Fmt:
  factorial.bux, fibonacci.bux, process.bux, os_time.bux

examples/factorial.bux

@@ -1,6 +1,6 @@
 // Factorial - Recursive function
-import Std::Io::{PrintLine, PrintInt};
-
+import Std::Io::{PrintLine};
+import Std::Fmt::{Fmt_Fmt2};
 
 func Factorial(n: int) -> int {
     if n <= 1 {
@@ -15,10 +15,7 @@ func Main() -> int {
     var i: int = 1;
     while i <= 10 {
         let fact: int = Factorial(i);
-        PrintInt(i);
-        PrintLine("! = ");
-        PrintInt(fact);
-        PrintLine("");
+        PrintLine(Fmt_Fmt2("{0}! = {1}", String_FromInt(i), String_FromInt(fact)));
         i = i + 1;
     }