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/source_location.bux

@@ -0,0 +1,13 @@
+// source_location.bux — Source position tracking
+module SourceLocation {
+
+struct SourceLocation {
+    line: uint32;
+    column: uint32;
+    offset: uint32;
+}
+
+func SourceLocation_New(line: uint32, column: uint32, offset: uint32) -> SourceLocation {
+    return SourceLocation { line: line, column: column, offset: offset };
+}
+}