chore(scaffold): initial SilverMetal program scaffold

Cross-platform privacy-hardening program. Two-layer product:
- SilverLABS Application Stack (cross-platform spine)
- Platform Hardening Profiles (per-OS, tier-honest)

Platforms: Linux (Debian/Kicksecure), Android (Pixel/Samsung/Moto/generic),
Windows (LTSC IoT), macOS (profile), iOS (MDM profile). Each flavour has
both a preflashed hardware SKU path and a self-apply "harden your existing
device" path.

Includes umbrella docs (README + threat-model, design-principles,
platform-matrix, roadmap, trust-model), per-platform and per-stack-
component README stubs, .gitignore, LICENSE.

Linux v1 ships first; Stack v1 = Browser + VPN + Sync.

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

stack/chat/README.md

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+# SilverChat
+
+**Status**: v1.1 (planning)
+
+End-to-end encrypted messenger. Ships post-MVP.
+
+## Approach (tentative — to be finalised before v1.1)
+
+Two candidate paths:
+
+1. **Matrix-based** — Synapse or Dendrite homeserver, custom client per platform. Pros: federated, mature, large existing ecosystem. Cons: metadata leakage in federation, complex protocol.
+2. **Signal-protocol-based** — fork the Signal codebase, run own server. Pros: gold-standard cryptography, simpler client. Cons: forking the Signal protocol is socially fraught; less feature-rich than Matrix.
+
+Decision documented in `docs/decisions/` once made.
+
+## Non-negotiables
+
+- Account-number-based identity (not phone, not email)
+- E2EE by default, no opt-out
+- Self-hostable server
+- No telemetry from client
+- Forward secrecy
+- Backup keys remain on user devices
+
+## Per-platform clients
+
+To be defined post-decision. Likely Tauri-based or per-platform-native depending on protocol choice.
+
+## Out of scope (for now)
+
+- Voice/video calling — v1.2+
+- Group sizes >100 — Matrix supports, but we may cap at 100 for v1.1 simplicity