fix(search): reject parentheses nested deeper than 64 levels

Deep nesting overflowed the stack of the recursive Pidgin parser (around
400-700 levels), an uncatchable crash. The depth is now checked on the token
stream before parsing and reported as a syntax error. Listed in PLAN 6.2 and
USAGE.

YKanBan.Tests/Search/SearchQueryParserTests.cs

@@ -227,6 +227,33 @@ public class SearchQueryParserTests
         AssertSyntaxError("a)");
     }
 
+    [TestMethod]
+    public void NestingAtTheDepthLimitParses()
+    {
+        int depth = SearchQueryParser.MaxNestingDepth;
+        AssertParsesTo(new SearchNode.FreeText("a"), new string('(', depth) + "a" + new string(')', depth));
+    }
+
+    [TestMethod]
+    public void NestingBeyondTheDepthLimitIsAnError()
+    {
+        int depth = SearchQueryParser.MaxNestingDepth + 1;
+        AssertSyntaxError(new string('(', depth) + "a" + new string(')', depth));
+    }
+
+    [TestMethod]
+    public void SequentialGroupsDoNotCountTowardsTheDepthLimit()
+    {
+        string groups = string.Join(" ", Enumerable.Repeat("(a)", SearchQueryParser.MaxNestingDepth + 1));
+        Assert.IsNotNull(SearchQueryParser.Parse(groups));
+    }
+
+    [TestMethod]
+    public void VeryDeepNestingIsAnErrorRatherThanACrash()
+    {
+        AssertSyntaxError(new string('(', 100_000) + "a" + new string(')', 100_000));
+    }
+
     [TestMethod]
     public void EmptyParensAreAnError()
     {