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

README.md

@@ -4,6 +4,14 @@ A lightweight DNS-over-HTTPS (DoH) forwarder written in Rust.
 
 Accepts DNS queries over traditional DNS (UDP/TCP) and HTTP DoH ([RFC 8484](https://www.rfc-editor.org/rfc/rfc8484)), then forwards them upstream via DoH (HTTPS POST) to configurable resolvers.
 
+```mermaid
+flowchart LR
+    client["DNS Client"] -- "UDP/TCP or HTTPS" --> forwarder["doh-forwarder"]
+    forwarder -- "HTTPS POST (DoH)" --> upstream["Upstream Resolvers\n(Cloudflare, Google, ...)"]
+    upstream -- "DNS response" --> forwarder
+    forwarder -- "DNS response" --> client
+```
+
 ## Why?
 
 See [docs/story.md](docs/story.md) for the full context — why this exists, what already works, and where this project is headed.