v0.3.0: restructure directories

- src/        ← compiler/selfhost/  (canonical Bux compiler)
- bootstrap/  ← compiler/bootstrap/ (Nim bootstrap)
- lib/        ← library/std/        (standard library)
- rt/         ← library/runtime/    (C runtime)
- tests/      ← compiler/tests/     (unit tests)
- Remove _selfhost/ (built into build/selfhost/ now)
- Update all path references (Makefile, cli.nim, cli.bux, docs)
- Bump version to 0.3.0

src/Main.bux

@@ -0,0 +1,23 @@
+// main.bux — Entry point for the Bux self-hosting compiler
+module Main {
+
+// C runtime for command-line args
+extern func bux_argc() -> int;
+extern func bux_argv(index: int) -> String;
+extern func bux_alloc(size: uint) -> *void;
+
+// Forward declaration from Cli module
+func Cli_Run(args: *String, argCount: int) -> int;
+
+func Main() -> int {
+    let count: int = bux_argc();
+    // Allocate array of String pointers
+    let args: *String = bux_alloc(count as uint * 8) as *String;
+    var i: int = 0;
+    while i < count {
+        args[i] = bux_argv(i);
+        i = i + 1;
+    }
+    return Cli_Run(args, count);
+}
+}