Adds a `version` field to the driverpb.HostInfo wire message + plumbs
it through to/from drivers.HostInfo in convert.go. Drivers report their
compile-time build version (e.g. "v0.6.0") via the existing HostInfo()
RPC ; weft surfaces it in the TUI / webui chrome.

Bumps weft-drivers floor to v0.4.0 (the same release that added
HostInfo.Version on the Go side).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>