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>

Cargo.toml

@@ -17,7 +17,7 @@ serde_json = "1"
 toml = "0.8"
 log = "0.4"
 env_logger = "0.11"
-reqwest = { version = "0.12", features = ["rustls-tls", "gzip"], default-features = false }
+reqwest = { version = "0.12", features = ["rustls-tls", "gzip", "http2"], default-features = false }
 hyper = { version = "1", features = ["client", "http1", "server"] }
 hyper-util = { version = "0.1", features = ["client-legacy", "http1", "tokio"] }
 http-body-util = "0.1"