Remove lossy default-action writes that can replace china-proxy
with gfwlist. Preserve legacy preference reads and derive the
default action from the selected preset.
Update tests to use explicit presets and verify identity,
legacy compatibility, and no-op reselection.
Add Mainland China proxy routing with independent rule workspaces.
Preserve existing routing choices through equivalent preset mappings.
BREAKING CHANGE: existing custom rules and enablement settings are
not migrated. Recreate them in the appropriate preset.
Qiu Yuzhou