A form needs two things the snapshot did not carry. The `user` layer tells it
which fields the user overrode — presence, not value equality, because an
override equal to the composition default is still an override — and `base`
is what a cleared field reverts to. `unset` is that clear, sharing `set`'s
queue, revision fence, and rejected-write recovery through one write path.
bindSettingsScope mirrors the Host-side settings owner seam in the browser:
one scope per namespace publishes a snapshot store (status, section value,
revision, writability, host/memory mode), validates sections against the
namespace's serialized wire schema via dsh-client-schema-form, and keeps the
controller's listener-before-read, revisioned serialized writes, latest-wins
publication, conflict recovery, and disposal quiescence. Theme, locale, and
busy-Enter services now take the scope as a constructor collaborator, which
removes the bindPersistence/syncPreference two-phase callback pair and the
defaulted no-op persist writers; hand-written wire guards fall away in favor
of the registered schema. test-runtime gains a stubSettingsScope double.
