v1.7.4: ship #275 (YouTube split) + #280 (negative-cache + pre-warm)

#275: youtube_via_relay no longer routes video/image CDNs through
Apps Script. The flag now correctly carves out only the API/HTML
hosts where Restricted Mode is enforced; video chunks come direct
from googlevideo.com (which was missing from the SNI rewrite list
entirely — fixed). Long videos no longer hit Apps Script's 6-min
execution cap, and single-chunk timeouts no longer abort playback.

#280: TunnelMux now caches "destination unreachable" responses from
the tunnel-node (Network is unreachable / No route to host) for 30
seconds, short-circuiting subsequent CONNECTs to that destination
with 502 (HTTP) or 0x04 (SOCKS5). Saves ~5 batches/second on
IPv6-only host probes. Startup pre-warm pool grew 12→24.

143/143 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mhrv-rs"
-version = "1.7.3"
+version = "1.7.4"
 edition = "2021"
 description = "Rust port of MasterHttpRelayVPN -- DPI bypass via Google Apps Script relay with domain fronting"
 license = "MIT"