fix: v1.9.14 — block_doh default upgrade-path regression (#773)

PR #763 added `block_doh: bool` with `#[serde(default)]`, which resolves to Rust's `Default::default() = false` for bool, not the `true` PR #763's docs intended. Existing configs upgrading from v1.9.10 → v1.9.13 had no block_doh field, so they got `false` paired with `tunnel_doh: true` (new default from #468) — every browser DoH lookup got tunneled through Apps Script, adding ~1.5s overhead per page load. User-perceived as "v1.9.13 is slower than v1.9.10" in #773.

Switched to a named-default function `default_block_doh() -> bool { true }` so the upgrade path actually delivers the fast block-then-system-DNS behaviour PR #763 advertised. Power users who specifically want browser DoH (with the latency cost) can still opt in with explicit `block_doh: false`.

Tests: 180 lib + 35 tunnel-node + UI release-mode build all green.

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

Cargo.lock

@@ -2222,7 +2222,7 @@ dependencies = [
 
 [[package]]
 name = "mhrv-rs"
-version = "1.9.13"
+version = "1.9.14"
 dependencies = [
  "base64 0.22.1",
  "bytes",