refactor(client): replace the per-field settings preference controller with a namespace settings scope

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.

.agents/notes/implemented/bug-fix/2026-08-06-host-backed-web-preferences.md

@@ -14,9 +14,9 @@ The first theme implementation moved only Appearance to Host settings but awaite
 
 The owning Host halves register three schemas: optional `locale.preference` (`zh` or `en`, where absence delegates to the browser), `ui-theme.preference` (`light`, `dark`, or `system`, default `system`), and `ui-conversation.busyEnter` (`queue` or `steer`, default `queue`). The local settings provider stores explicit choices in `$DSH_HOME/settings.yaml`, which resolves to `~/.dsh/settings.yaml` under the default home. The API proxy explicitly exposes all three namespaces beside the other Web settings; registration alone never crosses that configuration boundary.
 
-The client runtime provides one `bindSettingsPreference` lifecycle for scalar preferences. It installs `settings/changed` and `connection/reset` listeners before starting a background initial read, so no settings transport can block plugin activation and an invalidation cannot fall into a read-before-subscribe gap. Domain services publish their provisional defaults immediately—browser-derived locale, system theme, and Queue—then accept a validated Host value without writing it back.
+The client runtime provides one `bindSettingsScope` lifecycle per namespace — the browser mirror of the Host-side settings owner seam. It installs `settings/changed` and `connection/reset` listeners before starting a background initial read, so no settings transport can block plugin activation and an invalidation cannot fall into a read-before-subscribe gap, and it publishes a snapshot store (status, section value, revision, writability, host/memory mode) the domain service subscribes to. The default decoder validates each incoming section against the namespace's own serialized wire schema, rehydrated through dsh-client-schema-form, so domains carry no hand-written wire guards. Domain services take the scope as an ordinary constructor collaborator, publish their provisional defaults immediately—browser-derived locale, system theme, and Queue—then adopt an accepted Host section without writing it back; a service constructed without a scope (standalone dictionary or policy fixtures) simply stays process-local.
 
-User changes update the live service synchronously and queue a `settings.mutate` path operation. The controller serializes gestures, sends the latest known namespace revision as `expectedRevision`, records every successful revision, and lets only the latest write settlement republish live state. A rejected or failed latest write reloads Host state. Disposal rejects new work, skips queued operations, suppresses publication by the in-flight operation, and waits for that operation to settle before the plugin reaches quiescence.
+User changes update the live service synchronously and queue a `settings.mutate` path operation through `scope.set`. The scope serializes gestures, sends the latest known namespace revision as `expectedRevision`, records every successful revision, and lets only the latest write settlement republish live state. A rejected or failed latest write reloads Host state. Disposal rejects new work, skips queued operations, suppresses publication by the in-flight operation, and waits for that operation to settle before the plugin reaches quiescence.
 
 Remote browsers cannot call the loopback-only configuration API, so their preferences remain process-local. Dynamic third-party theme ids remain in-process extensions outside the built-in Host schema; removing one resets the live registry without replacing the last durable built-in preference.
 
@@ -28,7 +28,9 @@ Remote browsers cannot call the loopback-only configuration API, so their prefer
 
 **Await the initial read to avoid a provisional render.** Configuration availability is not a prerequisite for drawing the page. A background read may cause one live convergence, but it keeps failure isolated and preserves the existing browser/system/default fallbacks.
 
-**Give every domain its own settings controller.** The concurrency, revision, failure, invalidation, and disposal rules are identical; copying them already produced lifecycle drift in the theme implementation. Domain-owned schemas and decoders keep product policy out of the shared runtime.
+**Give every domain its own settings controller.** The concurrency, revision, failure, invalidation, and disposal rules are identical; copying them already produced lifecycle drift in the theme implementation. Domain-owned schemas keep product policy out of the shared runtime.
+
+**A per-field preference controller with paired sync/persist callbacks.** The first shared lifecycle synchronized one scalar field through a domain `sync` callback while the service wrote back through an injected `persist` callback. The mutual callbacks forced two-phase construction — a defaulted no-op writer later replaced via `bindPersistence` — every additional field of a namespace would have carried its own controller and whole-document read, and each domain re-declared a hand-written guard the registered wire schema already expresses. The namespace scope publishes a snapshot the service subscribes to and accepts writes directly, so the callback pair and the second construction phase do not exist.
 
 **Move every `localStorage` entry into settings.** Current session, drafts, panel disclosure, trajectory display state, and similar entries are browser-instance state rather than user configuration. Promoting them would synchronize transient navigation state across tabs and ports without a product contract.
 
@@ -38,4 +40,4 @@ Appearance, Language, and busy-Enter choices follow the DSH user home across rel
 
 Boot may briefly show the domain default before the background read settles. A transient read failure keeps that default or the last good in-process value; reconnect retries. A write rejection can visibly restore the durable preference after the immediate local change.
 
-Focused unit coverage pins schema registration, listener-before-read ordering, nonblocking activation, revisioned ordered writes, stale-response containment, failure recovery, disposal quiescence, and remote memory mode. The keyless Web settings scenario writes all three preferences through the UI, verifies the YAML document and empty legacy storage, reloads, and boots another Host on a distinct port against the same DSH home.
+Focused unit coverage pins schema registration, listener-before-read ordering, nonblocking activation, schema-validated section acceptance, revisioned ordered writes, stale-response containment, failure recovery, disposal quiescence, and remote memory mode. The namespace-granular scope also carries multi-field sections, so later configuration surfaces can ride the same lifecycle instead of hand-rolling describe/mutate synchronization. The keyless Web settings scenario writes all three preferences through the UI, verifies the YAML document and empty legacy storage, reloads, and boots another Host on a distinct port against the same DSH home.