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;
     }
     

examples/fibonacci.bux

@@ -1,6 +1,6 @@
 // Fibonacci - Recursive function with while loop
-import Std::Io::{PrintLine, PrintInt};
-
+import Std::Io::{PrintLine};
+import Std::Fmt::{Fmt_Fmt1};
 
 func Fibonacci(n: int) -> int {
     if n <= 1 {
@@ -15,8 +15,7 @@ func Main() -> int {
     var i: int = 0;
     while i < 10 {
         let fib: int = Fibonacci(i);
-        PrintInt(fib);
-        PrintLine("");
+        PrintLine(Fmt_Fmt1("{0}", String_FromInt(fib)));
         i = i + 1;
     }
     

examples/os_time.bux

@@ -1,37 +1,24 @@
 // Os & Time - System and time utilities
-import Std::Io::{PrintLine, PrintInt, PrintInt64};
+import Std::Io::{PrintLine};
 import Std::Os::{Os_ArgsCount, Os_Args, Os_GetEnv, Os_GetCwd};
 import Std::Time::{Time_NowMs, Time_SleepMs};
+import Std::Fmt::{Fmt_Fmt1, Fmt_FmtInt};
 
 func Main() -> int {
     PrintLine("=== OS Demo ===");
-    Print("Current directory: ");
-    PrintLine(Os_GetCwd());
-    
-    Print("Command-line args: ");
-    PrintInt(Os_ArgsCount());
-    PrintLine("");
-    
-    Print("HOME env: ");
-    PrintLine(Os_GetEnv("HOME"));
+    PrintLine(Fmt_Fmt1("Current directory: {0}", Os_GetCwd()));
+    PrintLine(Fmt_FmtInt("Command-line args: {0}", Os_ArgsCount() as int64));
+    PrintLine(Fmt_Fmt1("HOME env: {0}", Os_GetEnv("HOME")));
     
     PrintLine("\n=== Time Demo ===");
     let start: int64 = Time_NowMs();
-    Print("Start: ");
-    PrintInt64(start);
-    PrintLine(" ms");
-    
+    PrintLine(Fmt_FmtInt("Start: {0} ms", start));
     PrintLine("Sleeping 150ms...");
     Time_SleepMs(150);
     
     let end: int64 = Time_NowMs();
-    Print("End: ");
-    PrintInt64(end);
-    PrintLine(" ms");
-    
-    Print("Elapsed: ");
-    PrintInt64(end - start);
-    PrintLine(" ms");
+    PrintLine(Fmt_FmtInt("End: {0} ms", end));
+    PrintLine(Fmt_FmtInt("Elapsed: {0} ms", end - start));
     
     return 0;
 }

examples/process.bux

@@ -1,18 +1,16 @@
 // Process - Run external commands and capture output
-import Std::Io::{PrintLine, PrintInt};
+import Std::Io::{PrintLine};
 import Std::Process::{Process_Run, Process_Output};
+import Std::Fmt::{Fmt_Fmt1, Fmt_FmtInt};
 
 func Main() -> int {
     PrintLine("Process demo:");
     
     let output: String = Process_Output("echo Hello from subprocess");
-    Print("Captured: ");
-    PrintLine(output);
+    PrintLine(Fmt_Fmt1("Captured: {0}", output));
     
     let rc: int = Process_Run("true");
-    Print("Exit code: ");
-    PrintInt(rc);
-    PrintLine("");
+    PrintLine(Fmt_FmtInt("Exit code: {0}", rc));
     
     return 0;
 }