test: tag Raincloud corpus tests @Tag("raincloud"), excluded by default

[dfa1]

Summary

• A plain ./mvnw verify silently ran the full real-world Raincloud corpus whenever a developer had hydrated it locally: the two Raincloud tests only gated real work behind assumeTrue(Files.exists(manifestPath())), a no-op skip on CI but a surprise on a dev machine. RaincloudSizeComparisonIntegrationTest in particular does a full Parquet→Vortex re-encode (FSST training, etc.) per slug, costing a release preflight 15+ minutes of unexpected work.
• Tags both classes @Tag("raincloud"); Failsafe excludedGroups (bound to a new vortex.it.excludedGroups property, default raincloud) excludes them from test discovery entirely — stronger than assumeTrue's runtime skip.
• Failsafe 3.5.6 applies excludedGroups even to an explicitly named -Dit.test=Raincloud... class, and a literal pom value can't be cleared from the CLI — hence the property indirection. Opting in requires -Dvortex.it.excludedGroups= in addition to -Dit.test. Documented in CLAUDE.md and the hydrate script's usage/output hints.
• No CHANGELOG entry: test/tooling-only churn, no shipped-artifact or wire-behavior change.

Test plan

• Rebased onto current main (post-0.12.3 release + dependabot bumps + ParquetImporter fix) and re-verified after rebase
• ./mvnw verify -pl integration -am (corpus hydrated locally): neither Raincloud class discovered/run; 294 tests, 0 failures, ~1:25 total (previously 20+ min)
• Opt-in (-Dvortex.it.excludedGroups= -Dit.test=RaincloudConformanceIntegrationTest): full conformance run passes end-to-end against the real corpus
• ./mvnw test -pl integration -am: no regressions

🤖 Generated with Claude Code

[dfa1]

Ran the `review` skill against this PR before merging. It found the pom.xml/CLAUDE.md/hydrate-script core mechanism correct, but caught a real bug: two call sites documenting the old (now-broken) invocation weren't updated, and both test classes' javadoc had a wrong property name plus a claim that directly contradicts the pom.xml comment added in the same diff.

Fixed (amended into the PR commit):

1. .github/workflows/raincloud-conformance.yml — the weekly scheduled job ran -Dit.test=RaincloudConformanceIntegrationTest without -Dvortex.it.excludedGroups=; after merge it would have silently run 0 tests and stayed green, disabling the entire conformance signal.
2. docs/compatibility.md — same stale command.
   3/4. Both test classes' javadoc used the wrong property name (-DexcludedGroups=, a no-op) and RaincloudConformanceIntegrationTest's javadoc falsely claimed explicit -Dit.test selection overrides the tag exclusion.

Re-verified after fixing: plain `verify` still excludes both classes (294 tests, 0 failures, ~1min), and the corrected opt-in command (`-Dvortex.it.excludedGroups= -Dit.test=RaincloudConformanceIntegrationTest`) now actually runs the test (confirmed via the failsafe report: `Tests run: 1`).