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/output/05-custom-condition.txt

@@ -0,0 +1,19 @@
+The WeekendCondition from the 2025 article, run today (2026-10-11, a SUNDAY):
+  weeklyBackupStabilityTest() : PASSED
+
+The reusable version, @EnabledOnDays + DayOfWeekCondition, with the day pinned by the configuration parameter demo.today.
+
+demo.today=SATURDAY
+  noAnnotationRunsAlways() : PASSED
+  wednesdayOnly() : SKIPPED  (Today is SATURDAY, test runs on [WEDNESDAY])
+  weekendOnly() : PASSED
+
+demo.today=WEDNESDAY
+  noAnnotationRunsAlways() : PASSED
+  wednesdayOnly() : PASSED
+  weekendOnly() : SKIPPED  (Today is WEDNESDAY, test runs on [SATURDAY, SUNDAY])
+
+demo.today=FRIDAY
+  noAnnotationRunsAlways() : PASSED
+  wednesdayOnly() : SKIPPED  (Today is FRIDAY, test runs on [WEDNESDAY])
+  weekendOnly() : SKIPPED  (Today is FRIDAY, test runs on [SATURDAY, SUNDAY])