feat: bootstrap bun toolchain skeleton and smoke test

Co-authored-by: Eriyc <50216491+Eriyc@users.noreply.github.com>

README.md

@@ -1,3 +1,12 @@
 # rules_bun
 
-Bazel rules for bun
+Bazel rules for bun.
+
+## Current status
+
+Phase 1 bootstrap is in place:
+
+- Bun toolchain rule and provider (`/bun/toolchain.bzl`)
+- Platform-specific Bun repository downloads (`/bun/repositories.bzl`)
+- Toolchain declarations and registration targets (`/bun/BUILD.bazel`)
+- Smoke test for `bun --version` (`//tests/toolchain_test:bun_version_test`)