upstream: one absolute per-query budget across queueing and failover

waiting for a slot now spends the query budget; truncated attempts that
expire fault the budget, not the upstream, and are never attributed.
admission sweeps in priority order before blocking. forward zones spend
read_timeout_ms once across udp, truncation and tcp. adds
nxdns_upstream_budget_exhausted_total and a 64-upstream validation limit.

src/server/resolver_integration_test.zig

@@ -176,7 +176,7 @@ const EntryStorage = struct {
             .priority = priority,
             .enabled = true,
             .health = .init,
-            .sem = .{ .permits = self.slots.len },
+            .admission = .{ .permits = self.slots.len },
             .reuse_recoveries = &self.recoveries,
         };
     }