fix(mirror): seed "auto" so xlings' own region detection runs (remove curl probe)

Replaces the curl-based mcpp-side probe (per review: don't reinvent in mcpp;
mirror selection is xlings' job, and it already does it via tinyhttps).

Root cause of the repeated 75_index_status_offline.sh CI failures, now proven:
mcpp seeded a hardcoded "CN" into a fresh .xlings.json. xlings' normalize_mirror_
accepts only "GLOBAL"/"CN" as valid, so "CN" was used DIRECTLY (gitcode) —
bypassing xlings' own detect_install_mirror_(), which probes github vs gitcode
latency (tinyhttps::probe_latency) and picks the reachable/faster region. On a
GitHub-hosted (US) runner gitcode is slow/unreachable, so the cold index/sandbox
bootstrap failed and the index reported 'missing'.

Fix: seed "auto". normalize_mirror_("auto") -> nullopt (invalid) -> xlings
treats the mirror as unset -> runs detect_install_mirror_() -> picks GLOBAL on a
US runner, CN on a China link. mcpp no longer overrides xlings' region choice.
The existing-config guard (config.cppm: only seed when .xlings.json is absent)
already means an explicit `mcpp self config --mirror CN|GLOBAL` is never clobbered.

Empirically confirmed via the curl-probe build's verbose CI run: on the US runner
'probe github=70ms gitcode=1060ms -> GLOBAL' then patchelf/ninja bootstrap
succeeded and 75 PASSED — proving the mirror region is the cause. This commit
hands that region choice back to xlings instead of doing it in mcpp.

Author: Sunrisepeak

src/config.cppm

@@ -374,11 +374,13 @@ bool write_default_xlings_json(const std::filesystem::path& path,
     // construct a temporary Env with home = path.parent_path().
     mcpp::xlings::Env env;
     env.home = path.parent_path();
-    // No explicit --mirror: auto-detect the lower-latency reachable mirror
-    // instead of the historic hardcoded "CN" (which strands overseas users and
-    // GitHub-hosted CI). An explicit choice always wins (the else branch).
+    // No explicit --mirror: seed "auto" so xlings' own adaptive mirror module
+    // (latency-probed, with per-download failover) picks the best reachable
+    // host. The historic hardcoded "CN" FORCED the CN mirror and disabled that
+    // mechanism, stranding overseas users and GitHub-hosted CI on a
+    // slow/unreachable gitcode. An explicit --mirror always wins (else branch).
     if (mirror_override.empty())
-        mcpp::xlings::seed_xlings_json(env, pairs, mcpp::xlings::detect_best_mirror());
+        mcpp::xlings::seed_xlings_json(env, pairs);   // default = "auto"
     else
         mcpp::xlings::seed_xlings_json(env, pairs, mirror_override);
     return std::filesystem::exists(path);

src/xlings.cppm

@@ -225,24 +225,17 @@ int install_direct(const Env& env, std::string_view target, bool quiet = false);
 
 // Write .xlings.json seed file.
 //
-// The `mirror` parameter still defaults to "CN" for direct callers, but the
-// first-init seed path (config.cppm) now passes the result of
-// `detect_best_mirror()` when the user gave no explicit `--mirror` — i.e.
-// TODO(mirror-default) option (b) is implemented: a quick latency probe to the
-// GLOBAL vs CN hosts picks the faster reachable one, so overseas users and
-// GitHub-hosted CI no longer get stranded on the historical CN default. An
-// explicit `mcpp self config --mirror X` always wins (config priority).
+// The `mirror` default is "auto": xlings' own adaptive mirror module
+// (xlings.core.mirror.adaptive — latency-probed with per-download failover and
+// failure penalisation) then picks the best reachable host per download. This
+// replaces the historic hardcoded "CN", which FORCED the CN mirror and disabled
+// that mechanism — stranding overseas users and GitHub-hosted CI on a
+// slow/unreachable gitcode. An explicit `mcpp self config --mirror CN|GLOBAL`
+// still writes that fixed value (config priority). Mirror selection is xlings'
+// responsibility; mcpp just declines to override it by default.
 void seed_xlings_json(const Env& env,
                       std::span<const std::pair<std::string,std::string>> repos,
-                      std::string_view mirror = "CN");
-
-// Probe both index mirrors and return the lower-latency reachable one
-// ("GLOBAL" | "CN") — TODO(mirror-default) option (b). Used at first init when
-// the user gave no explicit --mirror; an explicit choice always wins (config
-// priority). Falls back to "GLOBAL" (reachable nearly everywhere) when neither
-// probe succeeds, since the historic "CN" default stranded overseas users and
-// GitHub-hosted CI behind a slow/unreachable mirror.
-std::string detect_best_mirror();
+                      std::string_view mirror = "auto");
 
 // Persist the xlings mirror selection in .xlings.json via xlings itself.
 int config_show(const Env& env);
@@ -1094,35 +1087,6 @@ void seed_xlings_json(const Env& env,
     write_file(path, json);
 }
 
-std::string detect_best_mirror() {
-    using namespace std::chrono;
-    // A short HEAD probe to each mirror host, timed by wall clock. Non-zero rc
-    // (DNS failure, connection refused, or the tight --max-time elapsing) means
-    // "unreachable". curl is present on every platform mcpp targets; if it is
-    // somehow absent both probes fail and we fall back to GLOBAL.
-    auto probe = [](std::string_view url) -> std::optional<double> {
-        // -I writes response headers to stdout; redirect both streams to null
-        // (cross-platform) so the probe stays silent — only its timing matters.
-        auto cmd = std::format("curl -s -I --max-time 3 {} {}",
-                               url, mcpp::platform::shell::silent_redirect);
-        auto t0 = steady_clock::now();
-        if (mcpp::platform::process::run_silent(cmd) != 0) return std::nullopt;
-        return duration<double>(steady_clock::now() - t0).count();
-    };
-    auto g = probe("https://github.com");
-    auto c = probe("https://gitcode.com");
-    if (g && c) {
-        std::string pick = (*g <= *c) ? "GLOBAL" : "CN";
-        mcpp::log::verbose("mirror", std::format(
-            "probe github={:.0f}ms gitcode={:.0f}ms -> {}", *g * 1000, *c * 1000, pick));
-        return pick;
-    }
-    if (g) { mcpp::log::verbose("mirror", "only github reachable -> GLOBAL"); return "GLOBAL"; }
-    if (c) { mcpp::log::verbose("mirror", "only gitcode reachable -> CN");   return "CN"; }
-    mcpp::log::verbose("mirror", "neither mirror reachable; defaulting -> GLOBAL");
-    return "GLOBAL";
-}
-
 int config_show(const Env& env) {
     auto cmd = std::format("{} config", build_command_prefix(env));
     return mcpp::platform::process::run_silent(cmd);