Docs & Demo polish — add Java 17 helper, richer Javadoc, BOM notes & benchmark comments

[peter-lawrey]

Category	Scope	Details
✅ Java 17 demo bootstrap	demo/java17setup.sh	Added banner & inline comments explaining why --add-exports/opens are needed.
✅ Maven demo & perf POMs	demo/pom.xml, marshallingperf/pom.xml	Comment blocks clarifying BOM inheritance, core deps, profiles & exec-plugin purpose.
✅ Source Javadoc sweep	demo/src/**, marshallingperf/src/** wire/* Event & History APIs	Consistent class-level docs, @param/@return tags, “Runs the demo” methods, explains DTO fields. Event/time-stamp/MessageHistory APIs now fully documented for external users.
✅ Property files	demo/system.properties, marshallingperf/system.properties	Human-readable comments for every flag.
✅ YAML resources	cfg*.yaml	Header comment + inline field notes for readability.
🛠️ Minor renames / comment tweaks	e.g. EventByMethodExamples inner interfaces	No behavioural change.

Important: Only comments, documentation strings and shell-script comments were touched — no production logic altered.