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/shared/README.md

@@ -0,0 +1,20 @@
+# Stack — Shared
+
+Common code shared across SilverLABS Stack components:
+
+- **Account SDK** — SilverLABS account-number issuance, auth, session management
+- **Crypto primitives** — vetted wrappers for Argon2id, AES-GCM, ChaCha20-Poly1305, X25519, Ed25519
+- **Branding tokens** — fonts, logos, colours, design tokens
+- **Update client** — common update-check + signature-verify code reused by all Stack apps
+
+To be populated as each Stack component reaches a point where shared code makes sense (typically v1 → v1.1 transition).
+
+## Language / runtime choice
+
+Default to **Rust** for shared crypto and account SDK code, with bindings/bridges per platform language:
+- Kotlin / Java for Android
+- Swift / Obj-C for Apple platforms
+- C# for Windows native integration where required
+- TypeScript bindings for browser extensions / web UI
+
+Rust gives memory safety, performance, and a single source of truth for the security-critical parts.