Add conditional module: JUnit conditional test execution on 6.1.3 (JDK 25)

Fixtures for the OS, JRE, system property, environment variable and EnabledIf
conditions plus a custom ExecutionCondition, run through the JUnit Platform
launcher (in-process and in child JVMs) to capture real skip reasons; skipped
versus aborted lifecycle; stacking, class-level skips and the deactivate switch;
the 2025 article's code compiled as published; and a javap comparison of the
condition package in 5.14.4 and 6.1.3.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01G8ikz8xdWuTP5yun8DZ1hk

conditional/scripts/run-all.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+# Regenerates every file under output/: 01-07 from the tests, 08-09 from the old-snippet and API-diff script.
+# No services needed. 01, 02 and 07 describe the machine they ran on; 05's first block depends on the day it runs.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+mvn -q -B test 2>&1 | grep -E "Tests run:|BUILD|FAIL|ERROR" || true
+bash scripts/compile-old-snippets.sh >/dev/null
+ls output