Cleaner platform abstraction (wasm vs wasi)

Right now we're using some std stuff related to paths that's not supported by WASM, along with some messy shims which are fine for testing but not so nice otherwise.

Questions:
- Should the `driver` crate target WASM or just WASI (which is std compatible)?
- If WASM:
- how can we avoid messy shims?
- what replaces `std::Path` on the web environment?

Start by inspecting the driver crate, its std-based path usage, and the existing testing shims. Determine whether the crate should target WASM or WASI, then investigate a suitable web-environment replacement for std::Path. Done means the platform abstraction and shim strategy are clearly defined and no longer rely on unsupported WASM path functionality.

Written by the indexing model from the issue text.