fix: children($var) domain compilation and resolver support

- Add children branch in convertDomain() alongside descendants —
  children() now correctly maps to parentVar: '$parent'
  instead of falling through to the default branch which reversed
  the parentVar/selector mapping.
- SelectorDomainResolver now handles parentVar domains with no
  CSS selector filter (children domain collects all registered
  subject IDs, then filters by parent via ancestor index).
- All 1072 tests pass across 14 packages.

packages/imhotep-dsl/src/compiler.ts

@@ -938,7 +938,7 @@ function convertDomain(domain: DslDomainRef): DomainRef {
 
   // Descendant domains use the first argument as parent and second as filter:
   // descendants($card, '.title') => parentVar: '$card', selector: '.title'
-  if (domain.kind === 'descendants') {
+  if (domain.kind === 'descendants' || domain.kind === 'children') {
     const parentVar = selectorFromVar ?? domain.selector
     const selector = extraArgFromVar ?? (domain as any).extraArg
     return {