docs: add Mermaid diagrams throughout docs

- plan.md: architecture flow diagram
- README.md: simple forwarder overview diagram
- story.md: DNS censorship and SOCKS5 proxy diagrams
- intra-comparison.md: all ASCII diagrams converted to Mermaid

docs/plan.md

@@ -17,17 +17,36 @@ A lightweight dual-protocol DNS forwarder written in Rust. It accepts DNS querie
 
 ## Architecture
 
-```
-┌─────────────┐      ┌──────────────────┐      ┌─────────────────┐
-│  DNS Client  │─────▶│  DoH Forwarder   │─────▶│  Upstream DoH   │
-│  (browser,   │ HTTPS│  (this project)  │ HTTPS│  (1.1.1.1,      │
-│   app)       │◀─────│  /dns-query      │◀─────│   8.8.8.8, …)   │
-└─────────────┘      │  /health         │      └─────────────────┘
-       │              └──────────────────┘
-       │ UDP/TCP              ▲
-       │ (port 5353)          │
-       └──────────────────────┘
-       System DNS (resolv.conf)
+```mermaid
+flowchart LR
+    subgraph clients["Clients"]
+        browser["Browser / App"]
+        os["OS DNS Client"]
+    end
+
+    subgraph forwarder["DoH Forwarder"]
+        doh["/dns-query (HTTP)"]
+        health["/health"]
+        dns["DNS Listener (UDP/TCP)"]
+        upstream["Upstream Client"]
+    end
+
+    subgraph upstreams["Upstream DoH Resolvers"]
+        cf["Cloudflare (1.1.1.1)"]
+        google["Google (8.8.8.8)"]
+        other["..."]
+    end
+
+    browser -- "HTTPS (RFC 8484)" --> doh
+    os -- "UDP/TCP :5353" --> dns
+    doh --> upstream
+    dns --> upstream
+    upstream -- "HTTPS POST" --> cf
+    upstream -- "HTTPS POST" --> google
+    upstream -- "HTTPS POST" --> other
+    cf -- "DNS response" --> upstream
+    google -- "DNS response" --> upstream
+    other -- "DNS response" --> upstream
 ```
 
 ## Tech Stack