Fix Bazel build output resolution under --symlink_prefix

[hartikainen]

Stacked on top of #53.

_build_multiple_targets currently joints the workspace directory with each output's path_prefix (bazel-out/...), which only resolves through the default bazel-out convenience symlink. Under a non-default --symlink_prefix (e.g. --symlink_prefix=.bazel/) that symlink doesn't exist, so the returned paths point nowhere and packaging fails.

This commit refactors the resolution to prefer file:// URI that Bazel emits in the BEP File messages. If I understand correctly, these are the authoritative on-disk paths, independent of --symlink_prefix. It falls back to <execution_root>/<path_prefix>/<name> for non-file:// URIs (e.g. bytestream:// outputs from remote builds that were downloaded locally via --remote_download_outputs=toplevel). Note this only resolves where an output lives; under --remote_download_minimal the bytes are never materialized locally, so downstream consumers still fail regardless of the path returned.

This also extracts the per-File resolution into a pure _resolve_output_path(file, exec_root) helper function and thus makes the logic unit-testable without shelling out to Bazel. The execution root is resolved lazily (and at most once) via functools.lru_cache to keep the unnecessary bazel info off the local-build path. A non-localhost URI authority is preserved as a //host/... UNC path rather than silently dropped. Adds tests covering file:// decoding (including percent-encoding and authorities) and the path_prefix fallback.

Implemented with the help of gemini-cli.