Add max_stack_depth to --debug-stats output (#639)

## Summary
- Tracks the high-water mark of the evaluation stack depth counter and
reports it as `max_stack_depth` in `--debug-stats` output.
- Useful for understanding how deep the call stack gets for a given
Jsonnet program and tuning `--max-stack` accordingly.

## Test plan
- [x] JVM tests pass (140/140)
- [x] Verified output manually with `--debug-stats`

Author: stephenamar-db

sjsonnet/src/sjsonnet/DebugStats.scala

@@ -16,6 +16,9 @@ final class DebugStats {
   var functionCalls: Long = 0
   var builtinCalls: Long = 0
 
+  // -- Stack --
+  var maxStackDepth: Int = 0
+
   // -- Comprehensions --
   var arrayCompIterations: Long = 0
   var objectCompIterations: Long = 0
@@ -39,6 +42,7 @@ final class DebugStats {
     sb.append(formatLine("lazy_created", lazyCreated))
     sb.append(formatLine("function_calls", functionCalls))
     sb.append(formatLine("builtin_calls", builtinCalls))
+    sb.append(formatLine("max_stack_depth", maxStackDepth))
     sb.append(formatLine("array_comp_iterations", arrayCompIterations))
     sb.append(formatLine("object_comp_iterations", objectCompIterations))
     sb.append(formatLine("files_parsed", filesParsed))

sjsonnet/src/sjsonnet/Evaluator.scala

@@ -33,6 +33,8 @@ class Evaluator(
 
   @inline private[sjsonnet] final def checkStackDepth(pos: Position): Unit = {
     stackDepth += 1
+    if (debugStats != null && stackDepth > debugStats.maxStackDepth)
+      debugStats.maxStackDepth = stackDepth
     if (stackDepth > maxStack)
       Error.fail("Max stack frames exceeded.", pos)
   }