Clarify and consolidate build-tag acceptance checks #1360

Build-tag computation is centralized in wheels.get_build_tag(), but the rules for accepting candidate wheels remain spread across:

• commands/build.py::_is_wheel_built()
• bootstrapper/_cache.py::_look_for_existing_wheel()
• bootstrapper/_cache.py::_download_wheel_from_cache()

These paths interpret tags differently. Bootstrap treats an empty expected tag as unconstrained. The build command defaults it to (0, "") for source-built packages and raises when a candidate has a higher build number with the same suffix.

PR #1359 addresses the immediate pre-built wheel failure. This follow-up should clarify the intended build-tag acceptance rules and determine which checks can be shared.

• In particular, establish how empty expected tags should be interpreted for source-built packages and whether higher build numbers should cause a cache miss or a fatal error.
• Any required differences between build and bootstrap should be explicit and documented.