feat: DNS-over-HTTPS (DoH) upstream forwarding (#14)

* feat: DNS-over-HTTPS upstream forwarding

Encrypt upstream queries via DoH — ISPs see HTTPS traffic on port 443,
not plaintext DNS on port 53. URL scheme determines transport:
https:// = DoH, bare IP = plain UDP. Falls back to Quad9 DoH when
system resolver cannot be detected.

- Upstream enum (Udp/Doh) with Display and PartialEq
- BytePacketBuffer::from_bytes constructor
- reqwest http2 feature for DoH server compatibility
- network_watch_loop guards against DoH→UDP silent downgrade
- 5 new tests (mock DoH server, HTTP errors, timeout)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* style: cargo fmt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add DoH to README — Why Numa, comparison table, roadmap

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/buffer.rs

@@ -21,6 +21,13 @@ impl BytePacketBuffer {
         }
     }
 
+    pub fn from_bytes(data: &[u8]) -> Self {
+        let mut buf = Self::new();
+        let len = data.len().min(BUF_SIZE);
+        buf.buf[..len].copy_from_slice(&data[..len]);
+        buf
+    }
+
     pub fn pos(&self) -> usize {
         self.pos
     }