[trav] Move rule selection to RectorRegistry

[TomasVotruba]

Moves per-node rule selection and filtering out of RectorNodeTraverser into RectorRegistry.

Changes

• RectorNodeTraverser now delegates to RectorRegistry::forNode() instead of its own getVisitorsForNode()/refreshPhpRectors(); PHP-version, composer-constraint and configuration filtering live in RectorRegistry::filterRectors().
• RectorRegistry::refreshRectors() now also clears the forNode cache, so a refreshed rule set is not shadowed by stale per-node cache entries (surfaced by the shared container across test classes).
• Dropped the dead refreshPhpRectors() call from AbstractRectorTestCase; the registry sync right below it already keeps rules in step.
• Tests migrated to the new API: StopTraverseOnTypeChangeTest and a rewritten RectorRegistryTest drive RectorRegistry; the orphaned RectorNodeTraverserTest (only tested the removed getVisitorsForNode) is folded into RectorRegistryTest.

[TomasVotruba]

Lets ship this 👍