feat(ext/sw): org_switch + org read messages (grant-filtered, offline-aware, cache cleared on lock)

Wires 4 SW messages to the org-vault.ts read functions (Task 5b):
- org_switch: opens org vault via openOrg(), caches OrgHandleState, sets
  session context; switching to 'personal' resets context without openOrg.
  Offline-aware: if openOrg throws a network error and a prior cached state
  exists, reuses it read-only (offline:true) via exported isNetworkError.
- org_list_items: returns cached, grant-filtered OrgManifestEntry[] (not
  personal ManifestEntry[]) via listOrgItems(requireCurrentOrgState()).
- org_get_item: decrypts a single org item via getOrgItem.
- org_list_collections: returns granted Collection[] via listOrgCollections.

Three-place rule satisfied for all 4 messages:
1. messages.ts PopupMessage union (4 variants)
2. messages.ts POPUP_ONLY_TYPES (4 entries)
3. popup-only.ts switch (4 case arms)

Cache lifecycle / use-after-free guard:
- clearOrgCache() added to org-handlers.ts; exports.
- Called in popup-only.ts 'lock' handler (before session.clearCurrent).
- Called in index.ts inactivity-timer onExpired (before clearCurrent).

Plan doc fixes:
- Line ~410: org_list_items return type corrected to OrgManifestEntry[].
- Line ~47: org_unwrap_key signature updated (no device_private_key_base64
  param — reads DEVICE_STATE internally).

9 new tests in org-handlers.test.ts (TDD: all watched fail before passing).
Full SW suite: 167 tests (17 files) green. build:all type-checks clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014s527M917W47LDrfQ4t47g

extension/src/shared/messages.ts

@@ -1,7 +1,7 @@
 import type {
   Item, ItemId, Manifest, ManifestEntry, VaultConfig, SetupState,
   DeviceSettings, GeneratorRequest, VaultSettings, AttachmentRef, Device,
-  FieldHistoryView, OrgConfigSummary,
+  FieldHistoryView, OrgConfigSummary, OrgManifestEntry, Collection,
 } from './types';
 
 // --- Session timeout config ---
@@ -69,7 +69,11 @@ export type PopupMessage =
   | { type: 'attach_vault'; config: VaultConfig; passphrase: string;
       referenceImageBytes: ArrayBuffer; deviceName: string }
   | { type: 'get_vault_status' }
-  | { type: 'org_list_configs' };
+  | { type: 'org_list_configs' }
+  | { type: 'org_switch'; context: string }
+  | { type: 'org_list_items' }
+  | { type: 'org_get_item'; id: string }
+  | { type: 'org_list_collections' };
 
 // --- Messages a content script may send ---
 
@@ -184,6 +188,7 @@ export const POPUP_ONLY_TYPES: ReadonlySet<PopupMessage['type']> = new Set([
   'generate_recovery_qr', 'unwrap_recovery_qr',
   'create_vault', 'attach_vault', 'get_vault_status',
   'org_list_configs',
+  'org_switch', 'org_list_items', 'org_get_item', 'org_list_collections',
 ] as PopupMessage['type'][]);
 
 export interface ExportBackupResponse extends Extract<Response, { ok: true }> {
@@ -227,6 +232,22 @@ export interface OrgListConfigsResponse extends Extract<Response, { ok: true }>
   data: OrgConfigSummary[];
 }
 
+export interface OrgSwitchResponse extends Extract<Response, { ok: true }> {
+  data: { context: string; offline: boolean };
+}
+
+export interface OrgListItemsResponse extends Extract<Response, { ok: true }> {
+  data: OrgManifestEntry[];
+}
+
+export interface OrgGetItemResponse extends Extract<Response, { ok: true }> {
+  data: Item;
+}
+
+export interface OrgListCollectionsResponse extends Extract<Response, { ok: true }> {
+  data: Collection[];
+}
+
 export const CONTENT_CALLABLE_TYPES: ReadonlySet<ContentMessage['type']> = new Set([
   'get_autofill_candidates', 'get_credentials', 'check_credential', 'blacklist_site',
   'capture_save_login',