test(customization): calibrate the one assertion made of wall clock

The chain case asserts two things, and only one of them was pinned. WHERE
the bound stops is already deterministic a case above — 998 links
admitted, 999 refused, one apart. What the clock adds is that the walk
bails rather than finishing and reporting, which no return value
distinguishes.

So the threshold is measured rather than chosen: 21 ms bounded on this
document, 61 ms on the first call because of JIT, against the 6.7 s the
unbounded walk took. Two seconds sits about thirty times above the one
and three below the other.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: cemililik

backend/tests/LearnStack.Tests.Unit/Infrastructure/Validation/JsonSchemaNetValidatorTests.cs

@@ -293,6 +293,16 @@ public void A_long_reference_chain_is_refused_without_paying_for_it()
         // Two thousand links, 64 KB, inside every other declared limit. Before the
         // graph was costed this was ADMITTED after 6.7 seconds of the profile's own
         // walking — LearnStack's code, not the library's. The bound makes it bail.
+        //
+        // <b>The clock is the second assertion, and it is calibrated rather than
+        // guessed.</b> WHERE the bound stops is pinned deterministically one case
+        // above — 998 links admitted, 999 refused, one link apart — so this case is
+        // only about the walk bailing instead of finishing and reporting afterwards,
+        // which no return value distinguishes. Measured on this document: 21 ms
+        // bounded (61 ms on the first call, which is JIT), against 6.7 s unbounded.
+        // The threshold sits about thirty times above the one and three times below
+        // the other, and a machine slow enough to cross it would have taken twenty
+        // seconds unbounded.
         var defs = Enumerable.Range(0, 2000)
             .Select(i => $"\"a{i}\":{{\"$ref\":\"#/$defs/a{i + 1}\"}}")
             .Append("\"a2000\":{\"type\":\"object\"}");
@@ -304,7 +314,8 @@ public void A_long_reference_chain_is_refused_without_paying_for_it()
             .SelectMany(messages => messages)
             .Should().Contain(m => m.Key == "lockey_schema_reference_graph_too_large");
         clock.Elapsed.Should().BeLessThan(TimeSpan.FromSeconds(2),
-            "the bound has to stop the walk, not merely report on it afterwards");
+            "the bound has to stop the walk, not merely report on it afterwards — "
+            + "measured at 21 ms bounded against 6.7 s unbounded");
     }
 
     [Fact]