docs(chat): adopt existing SilverVPN.Client.Chat as SilverChat — promote to v1

Inspection of ../SilverVPN/clients/SilverVPN.Client.Chat reveals a mature,
production-grade SilverChat implementation:

- Cross-platform MAUI client (Windows / macOS / Android / iOS)
- 13 ViewModels + 13 Views — feature-complete UX (contacts, conversations,
  group chat, invites, safety numbers, settings, login)
- Signal Protocol crypto: Double Ratchet, X3DH (PreKey + Identity stores),
  Safety Numbers, encrypted attachments
- VpnChatTransport — chat carried over the SilverVPN tunnel itself,
  eliminating third-party metadata exposure
- Server-side already in SilverVPN.Api: ChatHub (SignalR), ChatController,
  ChatAttachmentController, ContactsController
- Windows MSI installer wired (installer/silverchat/SilverChat.wxs)

Decision: adopt-as-is, do not duplicate. SilverChat is more advanced than
the v1.1 plan (which considered Matrix / Signal-fork) — three wins:
1. Signal Protocol natively, not a tentative fork
2. Chat over the VPN tunnel — better metadata hygiene
3. Cross-platform on day one

Changes:
- stack/chat/README.md rewritten as integration pointer (mirror of stack/vpn/)
- stack/README.md status table updated; SilverChat promoted v1.1 → v1
- docs/roadmap.md: new milestone 1.9 (Chat integration into Linux v1);
  Phase 1.1 alignment-review milestone removed (resolved by this finding);
  remaining 1.1 milestones renumbered
- root README.md: Stack table + Status table updated

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -35,7 +35,7 @@ Both lines ship the **SilverLABS Application Stack** — a suite of cross-platfo
 | **SilverBrowser** | v1 (Linux MVP) | De-Googled, telemetry-free, fingerprint-resistant browser |
 | **SilverVPN** | **Existing** — see [`SilverLABS/SilverVPN`](https://git.silverlabs.uk/SilverLABS/SilverVPN) | Always-on, no-logs VPN with our own infrastructure |
 | **SilverSync** | v1 (Linux MVP) | Private replacement for iCloud / Google Drive / OneDrive |
-| **SilverChat** | v1.1 (may overlap with `SilverVPN.Client.Chat`) | E2EE messenger |
+| **SilverChat** | **Existing** — `SilverVPN.Client.Chat`, Signal Protocol over VPN transport. Promoted from v1.1 to v1 | E2EE messenger |
 | **SilverDuress** | v1.1 | Duress password / panic-wipe / anti-coercion |
 | **SilverKeys** | v1.1 | Zero-knowledge password + 2FA manager |
 
@@ -57,6 +57,7 @@ Download the **free SilverLABS Stack** + the **SilverMetal OS or Enhanced packag
 | SilverMetal OS — Linux v1 | Phase 1 — moving to milestone 1.1 (build pipeline) |
 | SilverLABS Stack v1 (Browser + Sync) | Planning |
 | SilverVPN | Existing product, integration into v1 ISO planned |
+| SilverChat | Existing product (`SilverVPN.Client.Chat`); promoted to v1, integration into v1 ISO planned |
 | Other OS/Enhanced flavours | Planning, post-Linux v1 |
 
 See [`docs/roadmap.md`](docs/roadmap.md) for the milestone-driven plan.