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

_test_fmt2/bux.toml

@@ -0,0 +1 @@
+[Package]

_test_fmt2/src/Main.bux

@@ -0,0 +1,8 @@
+import Std::Io::{PrintLine};
+import Std::String::{String_Format1, String_FromInt};
+
+func Main() -> int {
+    let s: String = String_Format1("Hello, {0}!", "World");
+    PrintLine(s);
+    return 0;
+}