v0.6.0: performance pack — pool prewarm, SNI rotation, per-site stats, parallel dispatch

Tier-1 perf changes from the brainstorm, all on by default except where
they change semantics (parallel_relay is opt-in).

Connection pool pre-warm (domain_fronter.rs):
  On startup, open 3 TLS connections to Google edge in parallel and
  park them in the pool. First user request skips the ~300-500 ms
  handshake cost. Best-effort: warm failures are logged at debug and
  ignored. Triggered from ProxyServer::run() in a fire-and-forget
  tokio spawn.

SNI rotation (domain_fronter.rs):
  Replace the single sni_host String with a Vec<String> plus an atomic
  round-robin index. When front_domain is one of the known Google-edge
  subdomains, build_sni_pool() expands it to include the other four
  (www/mail/drive/docs/calendar.google.com), so outbound TLS connection
  counts get spread across names instead of concentrating on one. Custom
  front_domain values are preserved as the single entry (we can't verify
  siblings of a non-Google edge).

Expanded SNI-rewrite suffix list (proxy_server.rs):
  Added gstatic.com, googleusercontent.com, googleapis.com, ggpht.com,
  ytimg.com, blogspot.com, blogger.com to the list of domains routed
  directly via the Google-edge tunnel instead of through the Apps Script
  relay. Bigger bypass = less UA-locking, less quota burn on static CDN
  content.

Per-site stats (domain_fronter.rs + ui.rs):
  New HostStat struct {requests, cache_hits, bytes, total_latency_ns}
  tracked per URL host. Records on both cache hits and relay calls, not
  on SNI-rewrite bypasses (those never touch the fronter). UI renders
  a collapsible table under the existing stats grid with the top 60
  hosts sorted by request count, showing req count, cache hit %, bytes,
  avg latency ms.

Parallel script-ID dispatch (config.rs, domain_fronter.rs, ui.rs):
  New config field parallel_relay: u8 (default 0 = off). When >= 2 and
  there are enough non-blacklisted IDs, do_relay_with_retry fans out
  the request to N script instances concurrently via futures_util's
  select_ok, returns first success, cancels the rest. Kills long-tail
  latency when one Apps Script instance happens to be slow, at the
  cost of N× quota per request. UI exposes it as a DragValue 0-8.

TCP_NODELAY audit (proxy_server.rs):
  Added the missing set_nodelay(true) call on the SNI-rewrite outbound
  TCP stream. All six TcpStream::connect sites in the user traffic path
  now disable Nagle.

Expanded feature list in README, added futures-util dep, added unit
tests for extract_host and build_sni_pool.

Verified end-to-end locally:
- Pool pre-warm log line appears on startup: 'pool pre-warmed with 3
  connection(s)'.
- Static asset hit 3x: first = 2.2s (Apps Script), 2-3 = 6ms (cache).
- youtube.com / google.com: SNI-rewrite tunnel (unchanged).
- All 28 unit tests pass.

Deferred (not in this release, each needs its own cycle):
- uTLS / Chrome fingerprint mimicry (TLS stack swap)
- QUIC/HTTP3 transport (new transport)
- ETag / If-None-Match revalidation (needs cache schema change)
- JSON envelope gzip on request (needs Code.gs change)
- Firebase Cloud Functions as alt backend (new architecture)
- MSS clamp / TCP Fast Open (platform-specific, marginal)

README.md

@@ -261,6 +261,11 @@ This port focuses on the **`apps_script` mode** — the only one that reliably w
 - [x] Script IDs masked in logs (`prefix…suffix`) so `info` logs don't leak deployment IDs
 - [x] Desktop UI (egui) — cross-platform, no bundler needed
 - [x] Optional upstream SOCKS5 chaining for non-HTTP traffic (Telegram MTProto, IMAP, SSH…) so raw-TCP flows can be tunneled through xray / v2ray / sing-box instead of connecting directly. HTTP/HTTPS keeps going through the Apps Script relay.
+- [x] Connection pool pre-warm on startup (first request skips the TLS handshake to Google edge).
+- [x] Per-connection SNI rotation across a pool of Google subdomains (`www/mail/drive/docs/calendar.google.com`), so outbound connection counts aren't concentrated on one SNI.
+- [x] Optional parallel script-ID dispatch (`parallel_relay`): fan out a relay request to N script instances concurrently, return first success, kill p95 latency at the cost of N× quota.
+- [x] Per-site stats drill-down in the UI (requests, cache hit %, bytes, avg latency per host) for live debugging.
+- [x] OpenWRT / Alpine / musl builds — static binaries, procd init script included.
 
 Intentionally **not** implemented (rationale included so future contributors don't spend cycles on them):