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

MIGRATION_v0.3.0.sh

@@ -0,0 +1,38 @@
+#!/bin/bash
+# Bux v0.3.0 — Restructure directories
+set -e
+
+echo "=== Bux v0.3.0 Restructuring ==="
+
+# Step 1: Move compiler/selfhost → src
+echo "Moving compiler/selfhost → src/"
+git mv compiler/selfhost src
+
+# Step 2: Move compiler/bootstrap → bootstrap
+echo "Moving compiler/bootstrap → bootstrap/"
+git mv compiler/bootstrap bootstrap
+
+# Step 3: Move library/std → lib
+echo "Moving library/std → lib/"
+git mv library/std lib
+
+# Step 4: Move library/runtime → rt
+echo "Moving library/runtime → rt/"
+git mv library/runtime rt
+
+# Step 5: Move compiler/tests/*.nim → tests/
+echo "Moving compiler/tests/*.nim → tests/"
+for f in compiler/tests/*.nim; do
+    git mv "$f" tests/
+done
+
+# Step 6: Remove empty/obsolete directories
+echo "Removing _selfhost/"
+rm -rf _selfhost
+
+echo "Removing empty compiler/ and library/"
+rmdir compiler/tests 2>/dev/null || true
+rmdir compiler 2>/dev/null || true
+rmdir library 2>/dev/null || true
+
+echo "=== Done ==="