docs: comprehensive ARCHITECTURE, DEPLOYMENT, API, and THREAT_MODEL

ARCHITECTURE.md
- Full component map with ASCII diagram
- Complete session lifecycle (init + heartbeat + failure path)
- Cryptographic protocol spec (hash chain formula, canonical JSON)
- Stack machine instruction set table with stack effects
- Behavioral validation thresholds
- SQLite schema, threat model summary, module reference

DEPLOYMENT.md
- Build instructions (WASM + server + convenience script)
- native binary, systemd (with hardened sandbox notes), Docker
- nginx, Nginx Proxy Manager, and HAProxy reverse proxy configs
- Integration options (sidecar vs proxy-only)
- Full configuration table with all constants
- Observability (RUST_LOG levels), health check, security checklist

API.md
- Full /init and /hb request/response schemas with field tables
- Canonical signing payload specification
- Complete validation rules table (all 13 rejection conditions)
- Hash chain byte-level specification
- WASM exported function reference

THREAT_MODEL.md
- Four attacker profiles (script kiddie → sophisticated adversary)
- Eight attack vectors with mitigations (replay, forgery, hijack, DoS…)
- Explicit out-of-scope limitations
- Operational security notes (CORS, TLS, log level, SQLite)

docs/ARCHITECTURE.md

@@ -62,7 +62,7 @@ synchronisation burden alone makes scaled operation expensive.
 │                    │  ├ sign_message()            │     │
 │                    │  ├ compute_next_hash()       │     │
 │                    │  └ run_program()             │     │
-│                    └───────────────────────────────┘     │
+│                    └──────────────────────────────┘     │
 └─────────────────────────────────────────────────────────┘
                           │ HTTPS
 ┌─────────────────────────▼───────────────────────────────┐