feat(mining): pool registry, schema-aware stats fetcher, pool-select setting

Data + service layer for supporting multiple mining pools:
- pool_registry: KnownPool table (pool.dragonx.is + pool.dragonx.cc) carrying
  each pool's stratum host, xmrig algo, and stats-API schema; pure helpers
  findKnownPoolByUrl / resolvePoolAlgo / parsePoolHashrate / chooseWeightedPool
  (weighted-random by hashrate with incumbent stickiness).
- pool_stats_service: background, schema-aware hashrate fetcher (libcurl, TLS,
  browser UA for Cloudflare) modelled on XmrigUpdater.
- settings: pool_select_mode (manual | auto_balance), string-serialized.
- i18n strings + unit tests (both stats schemas, algo resolution, weighted pick).

Note: pool.dragonx.cc's stratum host is us.dragonx.cc:3333 — the .cc domain is
only the Cloudflare-proxied web/API host and does not accept stratum on :3333.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/config/settings.cpp

@@ -57,6 +57,25 @@ const char* liteServerSelectionPreferenceModeName(
     return "sticky";
 }
 
+Settings::PoolSelectMode parsePoolSelectMode(const json& value)
+{
+    if (!value.is_string()) return Settings::PoolSelectMode::Manual;
+    const std::string mode = value.get<std::string>();
+    if (mode == "auto_balance" || mode == "auto") {
+        return Settings::PoolSelectMode::AutoBalance;
+    }
+    return Settings::PoolSelectMode::Manual;
+}
+
+const char* poolSelectModeName(Settings::PoolSelectMode mode)
+{
+    switch (mode) {
+        case Settings::PoolSelectMode::Manual:      return "manual";
+        case Settings::PoolSelectMode::AutoBalance: return "auto_balance";
+    }
+    return "manual";
+}
+
 } // namespace
 
 std::string Settings::getDefaultPath()
@@ -257,6 +276,7 @@ bool Settings::load(const std::string& path)
         if (j.contains("pool_tls"))        pool_tls_       = j["pool_tls"].get<bool>();
         if (j.contains("pool_hugepages"))  pool_hugepages_ = j["pool_hugepages"].get<bool>();
         if (j.contains("pool_mode"))       pool_mode_      = j["pool_mode"].get<bool>();
+        if (j.contains("pool_select_mode")) pool_select_mode_ = parsePoolSelectMode(j["pool_select_mode"]);
         if (j.contains("mine_when_idle"))   mine_when_idle_ = j["mine_when_idle"].get<bool>();
         if (j.contains("xmrig_version"))    xmrig_version_  = j["xmrig_version"].get<std::string>();
         if (j.contains("mine_idle_delay"))  mine_idle_delay_= std::max(30, j["mine_idle_delay"].get<int>());
@@ -403,6 +423,7 @@ bool Settings::save(const std::string& path)
     j["pool_tls"]       = pool_tls_;
     j["pool_hugepages"] = pool_hugepages_;
     j["pool_mode"]      = pool_mode_;
+    j["pool_select_mode"] = poolSelectModeName(pool_select_mode_);
     j["mine_when_idle"] = mine_when_idle_;
     j["xmrig_version"]  = xmrig_version_;
     j["mine_idle_delay"]= mine_idle_delay_;