test(ci): sweep every module's effective javadoc config for silencers (#203)

[EtaCassiopeia]

Closes #203

Summary

The #200 poison canary only poisons rift-java-core, so it pins the ROOT pluginManagement. A per-module <failOnError>false</failOnError> override in another module leaves both the canary and the release build green while that module ships broken javadoc to Central.

This PR adds a new scripts/check-javadoc-strictness.py sweep that reads each module's effective javadoc configuration via help:effective-pom and rejects any silencer that could hide errors.

Evidence

The hole was real, proved first: Injecting a per-module <failOnError>false</failOnError> plus a broken {@link} into rift-java-jackson printed error: reference not found twice and still reported BUILD SUCCESS.

Configuration sweep: New scripts/check-javadoc-strictness.py reads help:effective-pom (the merged model—immune to whitespace, property indirection, or the setting living in another file) and rejects:

• failOnError (allowlist: only true)
• skip, skippedModules
• Any doclint other than all
• Any -Xdoclint option disabling a group
• Equivalent maven.javadoc.* / bare doclint properties (which need no <configuration> at all)

Self-test mode (--self-test): Runs in CI before the real sweep. Detects 14 silencer fixtures + 4 run_check round-trip cases + a strict fixture that must stay clean. Without it, a checker edit that stopped flagging anything would print "confirmed" forever.

Module requirement guard (--require): Names the 11 modules the JDK-21 + -DdualEmbedded reactor must contain, so a module silently dropping out fails the sweep instead of shrinking it.

Verification: Two review rounds found 5 blockers, each confirmed behaviourally before fixing:

• Property maven.javadoc.failOnError
• Property doclint
• Config value doclint=syntax
• Config value -Xdoclint:all,-missing,-reference
• Typo'd --require flag that silently no-op'd

The existing #200 canary is untouched (pure addition) and still passes. The two guards are complementary: canary = behaviour, sweep = configuration.

Known Limitation

The sweep reads POM-reachable configuration and cannot see a -Dmaven.javadoc.failOnError=false added to a workflow's own mvn command line. This is documented in the script header and CONTRIBUTING.md, and being filed as a separate issue.

Also pins maven-help-plugin to avoid prefix-invoked goals resolving latest-on-Central, keeping the sweep reproducible.