fix: DNS петля + watchdog автоисправление + централизация file_is_usable

Ключевые изменения:

1. core/helpers.uc: file_is_usable(path, min_bytes) + file_size_value(path)
   для единообразной проверки файлов (минимальный размер порог).

2. singbox/dns.uc: WAN fallback DNS серверы (fallback_wan_main) теперь
   используют прямой UDP без detour — исправлена петля когда
   DNS шёл через мёртвую прокси-секцию.
   Health probes для ВСЕХ DNS серверов (включая DoH) идут direct —
   failover может определить доступность DNS серверов.

3. watchdog.uc: ai_heal_empty_sections() — автообнаружение пустых
   proxy-секций с подписками → запуск subscription_update_async.
   ai_heal_dns_loop() — при обнаружении DNS-петли (dead proxy +
   dead DNS) → временно отключает DNS detour → загружает подписку →
   восстанавливает настройки.

4. Единая file_is_usable: nft/apply.uc, generator_routes.uc,
   components/{updates,action}.uc, config/validator.uc,
   subscription/cache.uc — все используют helpers.file_is_usable.

5. Тест tests/file_usable.sh для проверки helpers CLI.

6. Исправлен components_updater_job.sh mock lib — добавлен core/helpers.uc

tests/components_updater_job.sh

@@ -191,6 +191,7 @@ package_runtime_bin="$WORK_DIR/package-runtime-bin"
 mkdir -p "$package_runtime_lib/components" "$package_runtime_lib/core" "$package_runtime_lib/singbox" "$package_runtime_bin"
 cp "$UPDATER" "$package_runtime_lib/components/updater.uc"
 cp "$TACHYON_LIB/core/common.uc" "$package_runtime_lib/core/common.uc"
+cp "$TACHYON_LIB/core/helpers.uc" "$package_runtime_lib/core/helpers.uc"
 cat >"$package_runtime_lib/core/constants.uc" <<'UCODE'
 function module_exports() {
   return {};
@@ -380,6 +381,7 @@ cp "$UPDATES_UC" "$fake_lib/components/updates.uc"
 cp "$TACHYON_LIB/config/connections.uc" "$fake_lib/config/connections.uc"
 cp "$TACHYON_LIB/core/uci.uc" "$fake_lib/core/uci.uc"
 cp "$TACHYON_LIB/core/common.uc" "$fake_lib/core/common.uc"
+cp "$TACHYON_LIB/core/helpers.uc" "$fake_lib/core/helpers.uc"
 cat >"$fake_lib/components/action.uc" <<'UCODE'
 #!/usr/bin/env ucode
 if ((ARGV[0] || "") == "component-action") {

tests/file_usable.sh

@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+set -eo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+HELPERS_UC="$ROOT_DIR/tachyon/files/usr/lib/core/helpers.uc"
+TACHYON_LIB="$ROOT_DIR/tachyon/files/usr/lib"
+TMP_DIR="$ROOT_DIR/tmp_file_usable_test"
+
+rm -rf "$TMP_DIR"
+mkdir -p "$TMP_DIR"
+
+ucode() {
+  command ucode -L "$TACHYON_LIB" "$@"
+}
+
+fail() {
+  printf 'FAIL: %s\n' "$1" >&2
+  exit 1
+}
+
+assert() {
+  local expected="$1"
+  local actual="$2"
+  local label="$3"
+  [ "$actual" = "$expected" ] || fail "$label: expected '$expected', got '$actual'"
+}
+
+# --- file-is-usable CLI tests ---
+# Nonexistent file
+assert 1 "$(ucode "$HELPERS_UC" file-is-usable "$TMP_DIR/nonexistent.srs" 100 2>/dev/null; echo $?)" "nonexistent file not usable"
+
+# Empty file
+touch "$TMP_DIR/empty.srs"
+assert 1 "$(ucode "$HELPERS_UC" file-is-usable "$TMP_DIR/empty.srs" 100 2>/dev/null; echo $?)" "empty file not usable"
+
+# 1-byte file
+printf x > "$TMP_DIR/1byte.srs"
+assert 1 "$(ucode "$HELPERS_UC" file-is-usable "$TMP_DIR/1byte.srs" 100 2>/dev/null; echo $?)" "1-byte file not usable at min 100"
+
+# 49-byte file
+dd if=/dev/urandom of="$TMP_DIR/49byte.srs" bs=1 count=49 2>/dev/null
+assert 1 "$(ucode "$HELPERS_UC" file-is-usable "$TMP_DIR/49byte.srs" 50 2>/dev/null; echo $?)" "49-byte file not usable at min 50"
+assert 0 "$(ucode "$HELPERS_UC" file-is-usable "$TMP_DIR/49byte.srs" 0 2>/dev/null; echo $?)" "49-byte file usable at min 0"
+
+# 50-byte file
+dd if=/dev/urandom of="$TMP_DIR/50byte.srs" bs=1 count=50 2>/dev/null
+assert 0 "$(ucode "$HELPERS_UC" file-is-usable "$TMP_DIR/50byte.srs" 50 2>/dev/null; echo $?)" "50-byte file usable at min 50"
+
+# 100-byte file
+dd if=/dev/urandom of="$TMP_DIR/100byte.srs" bs=1 count=100 2>/dev/null
+assert 0 "$(ucode "$HELPERS_UC" file-is-usable "$TMP_DIR/100byte.srs" 100 2>/dev/null; echo $?)" "100-byte file usable at min 100"
+assert 1 "$(ucode "$HELPERS_UC" file-is-usable "$TMP_DIR/100byte.srs" 200 2>/dev/null; echo $?)" "100-byte file not usable at min 200"
+
+# No min_bytes = always usable if exists
+assert 0 "$(ucode "$HELPERS_UC" file-is-usable "$TMP_DIR/1byte.srs" 2>/dev/null; echo $?)" "1-byte file usable with no min"
+
+# --- file-size CLI tests ---
+assert "-1" "$(ucode "$HELPERS_UC" file-size "$TMP_DIR/nonexistent.srs" 2>/dev/null)" "nonexistent file size is -1"
+assert "0" "$(ucode "$HELPERS_UC" file-size "$TMP_DIR/empty.srs" 2>/dev/null)" "empty file size is 0"
+assert "49" "$(ucode "$HELPERS_UC" file-size "$TMP_DIR/49byte.srs" 2>/dev/null)" "49-byte file size is 49"
+assert "100" "$(ucode "$HELPERS_UC" file-size "$TMP_DIR/100byte.srs" 2>/dev/null)" "100-byte file size is 100"
+
+# Clean up
+rm -rf "$TMP_DIR"
+
+printf 'file_usable checks passed\n'