Port Bun import-attributes loader/build matrix

Focused child of #66. Activate the byte-identical js/bun/import-attributes/import-attributes.test.ts corpus by fixing thread-safe stable resolver string storage, safe bundler parser control flow, cwd-relative virtual eval modules, and automatic full-VM routing. Acceptance: exact file 12/12 and full home_test gate. Work remains open until the verified commit is pushed.

Completed in 386526f, pushed directly to main. Home now provides mutex-serialized pointer-stable BSSStringList storage with owned overflow buffers; bundler parse tasks cannot panic by reading a non-AST union variant; inline eval uses the Bun-compatible cwd/[eval] virtual source so relative imports, import attributes, require, and query loaders resolve from cwd; and this corpus is routed automatically through the native VM. The upstream file is byte-identical. Validation: exact corpus 12 pass, 0 fail, 8 snapshots, 91 assertions; debug build passes; full home_test gate passes; formatting and diff checks pass. The unrelated ts-checker edit and untracked artifacts were excluded.