fix(settings): harden seam and provider per review findings

Confirmed and fixed, each with a regression test that failed first:

- Concurrent update() lost patches (merge over one stale snapshot):
  per-namespace serialized write queues; a failed write cannot poison
  the queue for later writers.
- Fixed-name .tmp write followed planted symlinks and kept stale modes:
  random-suffix sibling, exclusive-create (wx), 0600, cleanup on
  failure, then rename.
- A throwing settings/updated listener escaped commit and permanently
  wedged the provider reload chain (rejected refreshTask): commit now
  contains listener failures (INVARIANT-coded errors still propagate),
  async watcher rejections are adopted and contained
  (watch callbacks are officially void | Promise<void>), and the
  provider chains refreshes on a settled tail with an error log.
- No way to remove a user override: scope/service replace(section)
  sets the user section wholesale; replace({}) re-inherits base and
  schema defaults.
- The three-primitive provider contract did not hold (base never
  called load()): the base Service.init loads and publishes once;
  settings-local delegates via yield* super[Service.init]().
- Dispose did not quiesce: teardown flags closed, closes the watcher,
  then awaits queued/in-flight reloads; closed is re-checked across
  await points.
- Invariant now checks the authoritative relation with the seam's own
  deepEqualJson: emitted next must equal settings.get(ns), and
  next/prev must differ structurally (cosmokit dependency dropped).
- New docs/core-data-structures/settings.{md,zh.md} with type-equiv
  blocks + manifest entries; catalog types moved from exemptions to
  LINK_MAP; website page registered.

Both packages stay at per-file 100% coverage.

docs/core-data-structures/settings.md

@@ -0,0 +1,94 @@
+# User Settings
+
+English | [中文](settings.zh.md)
+
+The user-settings seam of [dsh-settings](../../packages/settings/settings) holds one user-owned document of per-namespace sections and resolves each registered namespace as schema defaults, then the registrant's composition `base`, then the user section. Providers such as [dsh-settings-local](../../packages/settings/settings-local) store the raw document and push external edits; consumer plugins register a schema and read or observe the resolved value. Composition config stays in `cordis.yml` — a namespace carries only the user-editable subset.
+
+Source: [`packages/settings/settings/src/index.ts`](../../packages/settings/settings/src/index.ts)
+
+## Identity
+
+A namespace names one plugin-owned section of the user document. The brand keeps namespaces from mixing with other cross-boundary ids; construction validates the lowercase kebab-case shape.
+
+```ts type-equiv
+/** Nominal id of one registered settings namespace. */
+type SettingsNamespace = Branded<'SettingsNamespace'>
+```
+
+## Registration
+
+Registration binds a schemastery schema to a namespace on the calling plugin's fiber — disposing that fiber removes the namespace and its observers. The options carry the composition layer and the owner's effect timing.
+
+```ts type-equiv
+/** Registration options beyond the namespace schema. */
+interface SettingsRegisterOptions<T> {
+  /** Composition-layer values resolved below the user layer (entry-config subset). */
+  base?: Partial<T>
+  /** Owner's effect timing, surfaced to configuration UIs; defaults to `live`. */
+  applies?: SettingsApplies
+}
+```
+
+`applies` is a UI hint, not a mechanism: a `restart` owner simply never watches, so its value is read once at construction and configuration surfaces can badge the pending change.
+
+```ts type-equiv
+/** When a namespace's changes take effect for its owner. */
+type SettingsApplies = 'live' | 'restart'
+```
+
+## Owner scope
+
+The scope is the owner-facing handle. `update` merges a sparse patch over the user section only (never into `base`); `replace` sets the section wholesale, which is the removal/reset path — keys absent from the replacement re-inherit `base` and schema defaults. Writes to one namespace are serialized in call order, and resolved values are deep-frozen snapshots.
+
+```ts type-equiv
+/** Owner-facing handle for one registered namespace. */
+interface SettingsScope<T> {
+  /** Current resolved value: schema defaults, then `base`, then the user layer. */
+  get(): T
+  /**
+   * Observe committed changes to this namespace's resolved value. A callback
+   * may be async; a rejection is contained and logged like a sync throw.
+   * @param callback - invoked after each commit with the next and previous values.
+   * @returns the disposer removing this observer.
+   */
+  watch(callback: (next: T, prev: T) => void | Promise<void>): () => void
+  /**
+   * Merge a partial patch into this namespace's user layer and persist it.
+   * @param patch - plain-object patch over the user section.
+   */
+  update(patch: object): Promise<void>
+  /**
+   * Replace this namespace's user section wholesale; absent keys re-inherit
+   * the composition `base` and schema defaults (`replace({})` resets all).
+   * @param section - the complete next user section.
+   */
+  replace(section: object): Promise<void>
+}
+```
+
+## Descriptors
+
+`describe()` serializes every registered namespace for configuration surfaces: the schemastery `toJSON()` envelope drives schema-rendered forms, and the resolved value fills them.
+
+```ts type-equiv
+/** One registered namespace as surfaced to configuration UIs. */
+interface SettingsDescriptor {
+  /** The registered namespace. */
+  ns: SettingsNamespace
+  /** Serialized schemastery schema (`schema.toJSON()`). */
+  schema: unknown
+  /** Current resolved value. */
+  value: unknown
+  /** Owner's declared effect timing. */
+  applies: SettingsApplies
+}
+```
+
+## Change commits
+
+Every committed change — an in-process write or an externally observed provider edit — emits `settings/updated (ns, next, prev, source)` after the new value is authoritative, and never when the resolved value is deep-equal. The source tag separates the two entry paths.
+
+```ts type-equiv
+/** Origin of one committed settings change. */
+type SettingsUpdateSource = 'update' | 'provider'
+```