harktech: move vendored mdns into deps_src/ per repo convention

Per @SoftFever review on #13752, third-party vendored libraries belong in
deps_src/ alongside expat, imgui, hidapi, etc.

- All 5 files (mdns.{h,c}, cxmdns.{h,cpp}, NOTICE.md) move from
  src/slic3r/Utils/mdns/ to deps_src/mdns/.
- deps_src/mdns/CMakeLists.txt builds mdns as a STATIC library and scopes
  the MSVC Iphlpapi/Ws2_32 link requirement to that target instead of
  libslic3r_gui's global MSVC block.
- deps_src/CMakeLists.txt gains add_subdirectory(mdns).
- src/slic3r/CMakeLists.txt drops the inline source listings and links
  libslic3r_gui against the new mdns target; MSVC block keeps only
  Setupapi.lib.
- src/slic3r/Utils/CrealityHostDiscovery.cpp #include updated to use the
  include dir exposed by the new mdns target.

Verified by a clean Linux build (orca-slicer links successfully).

deps_src/mdns/NOTICE.md

@@ -0,0 +1,39 @@
+# mDNS / DNS-SD library
+
+The four files in this directory implement mDNS / DNS-SD lookup and are
+vendored from third-party sources:
+
+## mdns.h, mdns.c
+
+mDNS / DNS-SD lookup library by Mattias Jansson. Originally released to
+the public domain at https://github.com/mjansson/mdns.
+
+The exact files here were taken from CrealityOfficial/CrealityPrint
+v7.1.1, which split the upstream header-only library into separate
+declaration (mdns.h) and implementation (mdns.c) files.
+
+- Source: https://github.com/mjansson/mdns
+- License: Public domain (no restrictions on use)
+
+## cxmdns.h, cxmdns.cpp
+
+Thin C++ wrapper over mdns.{h,c} that exposes a single function:
+
+  std::vector<machine_info> syncDiscoveryService(
+      const std::vector<std::string>& prefix);
+
+It sends a DNS-SD meta-discovery query (`_services._dns-sd._udp.local.`),
+listens for ~5 seconds, and returns `{ip, service_name}` for every
+service announcement whose name contains any of the given prefixes.
+
+OrcaSlicer uses this to find Creality K-series printers on the LAN
+(service-name prefix "Creality"), since K-series firmware announces
+each printer under a per-device-unique service type
+`_Creality-<MAC-derived-hex>._udp.local.` that no fixed-name query can
+target.
+
+- Source: CrealityOfficial/CrealityPrint v7.1.1
+  `src/slic3r/GUI/print_manage/utils/cxmdns.{h,cpp}`
+- License: GNU AGPL-3.0 (compatible with OrcaSlicer's AGPL-3.0; see
+  top-level LICENSE.txt)
+- Imported: 2026-05-19