feat(us-027/us-028): throughput-optimized routing + correctness tests

_select_route now prefers nodes with higher effective throughput
(benchmark_tokens_per_sec / (queue_depth + 1)) when multiple nodes
cover the same interval, breaking the tie after max-reach selection.
TorchNodeServer.route_timeout property added for external inspection.

Tests added:
- _select_route with overlapping shards (A:0-22, B:20-24) → correct X-Meshnet-Start-Layer protocol
- _select_route with gap → clear error message
- Fast node beats slow node when shards are equal
- Busy fast node vs idle slow node (queue_depth factor)
- Two-StubNodeServer integration via TrackerServer
- route_timeout property exposed on TorchNodeServer

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/tracker/meshnet_tracker/server.py

@@ -292,12 +292,22 @@ class _NodeEntry:
         self.pending_new_assignment: dict | None = None
 
 
+def _effective_throughput(node: "_NodeEntry") -> float:
+    """Effective tokens/s accounting for current queue depth."""
+    return node.benchmark_tokens_per_sec / (node.queue_depth + 1)
+
+
 def _select_route(
     nodes: list[_NodeEntry],
     required_start: int,
     required_end: int,
 ) -> tuple[list[_NodeEntry], str]:
-    """Greedy interval-cover. Returns (ordered route, error_message)."""
+    """Greedy interval-cover biased toward fast, lightly-loaded nodes.
+
+    Among nodes that equally advance coverage, prefer the one with higher
+    effective throughput: benchmark_tokens_per_sec / (queue_depth + 1).
+    Tiebreak: higher shard_end (fewer hops).
+    """
     candidates = sorted(
         [node for node in nodes if node.shard_start is not None and node.shard_end is not None],
         key=lambda n: (n.shard_start, -n.shard_end),  # type: ignore[operator]
@@ -309,7 +319,11 @@ def _select_route(
         best: _NodeEntry | None = None
         for node in candidates:
             if node.shard_start <= covered_up_to + 1 and node.shard_end > covered_up_to:
-                if best is None or node.shard_end > best.shard_end:
+                if best is None:
+                    best = node
+                elif node.shard_end > best.shard_end:
+                    best = node
+                elif node.shard_end == best.shard_end and _effective_throughput(node) > _effective_throughput(best):
                     best = node
         if best is None:
             missing = covered_up_to + 1