fix: validate deep binary expressions iteratively

[martinfrancois]

Summary

Process very deep binary-expression trees without overflowing the JVM stack.

Fixes #6804.

Problem

OpenJDK's valid DeepStringConcat.java contains about 32,000 binary operators in one expression. Spoon recursively visited one operator per JVM frame. It first overflowed while checking parent links and then overflowed while running processors.

Change

• Use an explicit stack when the concrete parent checker visits consecutive binary operands.
• Use the same approach in the concrete processing visitor.
• Preserve Spoon's existing child order and callbacks before and after each child.
• Preserve the normal recursive behavior for subclasses that override scanner hooks.
• Avoid recursive pretty-printing while reporting malformed parent links.
• Restore the parent checker's previous root state after a direct accept call.

Validation

• Added deep-tree tests for parent checking and processor traversal.
• Added tests for child order, synchronous hooks, subclass overrides, malformed-tree diagnostics, and repeated checker use.
• mvn -q -Dtest=spoon.reflect.visitor.AstParentConsistencyCheckerTest,spoon.support.visitor.ProcessingVisitorTest,spoon.FluentLauncherTest test passes all 12 tests.
• mvn -q -Dtest=spoon.test.architecture.SpoonArchitectureEnforcerTest test passes all 9 tests. The two focused commands therefore run 21 tests in total.
• StoneDetector processes OpenJDK's DeepStringConcat.java: exit 0, AST 1/1, and no reported errors.
• All 14 GitHub checks pass, including the Java 17, 21, and 25 test matrix, coverage, Javadoc quality, code generation, reproducibility, Qodana, and QDJVM.