perf: cache compute.host_ip + add UDPLink.create timing log

host_ip resolved socket.gethostbyname on every access with no cache.
get_ip_on_same_subnet touches host_ip 2-4 times per link, so opening a
2500-link project issued thousands of blocking DNS calls inline on the
event loop — freezing all concurrent link coroutines each time. This is
the most likely cause of the 12-link/s throughput (1000x below what
Pool(concurrency=100) should deliver) and the burst+pause pattern.

- compute.host_ip: cache the resolution in _host_ip_cache, invalidate
  on host setter change
- UDPLink.create: timing log splitting get_ip / ports / nio so the next
  project-open confirms where time actually goes

gns3server/controller/udp_link.py

@@ -17,6 +17,8 @@
 
 
 import asyncio
+import time
+import logging
 
 from .controller_error import ControllerError, ControllerNotFoundError
 from .link import Link, _UNSET
@@ -32,6 +34,9 @@ _MARKER_CAPABLE_TYPES = frozenset({
 })
 
 
+log = logging.getLogger(__name__)
+
+
 class UDPLink(Link):
     def __init__(self, project, link_id=None):
         super().__init__(project, link_id=link_id)
@@ -96,10 +101,12 @@ class UDPLink(Link):
         port_number2 = self._nodes[1]["port_number"]
 
         # Get an IP allowing communication between both host
+        _t0 = time.perf_counter()
         try:
             (node1_host, node2_host) = await node1.compute.get_ip_on_same_subnet(node2.compute)
         except ValueError as e:
             raise ControllerError(f"Cannot get an IP address on same subnet: {e}")
+        _t1 = time.perf_counter()
 
         # Reserve a UDP port on both sides in parallel. Pre-allocated ports
         # (used during batch project loading) are popped from memory; otherwise
@@ -114,6 +121,7 @@ class UDPLink(Link):
         self._node1_port, self._node2_port = await asyncio.gather(
             _allocate_port(node1.compute), _allocate_port(node2.compute)
         )
+        _t2 = time.perf_counter()
 
         node1_filters, node2_filters = self._get_node_filters(node1, node2)
         node1_markers, node2_markers = self._get_node_markers(node1, node2)
@@ -171,6 +179,11 @@ class UDPLink(Link):
             if cleanup:
                 await asyncio.gather(*cleanup, return_exceptions=True)
             raise errors[0]
+        _t3 = time.perf_counter()
+        log.info(
+            "UDPLink.create timing get_ip=%.3fms ports=%.3fms nio=%.3fms total=%.3fms",
+            1000 * (_t1 - _t0), 1000 * (_t2 - _t1), 1000 * (_t3 - _t2), 1000 * (_t3 - _t0)
+        )
         self._created = True
         # New links automatically inherit every active project-level marker
         # definition so the user doesn't have to reconfigure.