fix(core): reenter:true targeting a child no longer reenters sibling parallel regions

[JSap0914]

Summary

Closes #5162

When a compound state uses reenter: true on a transition that targets one of its own children (e.g. target: '.a'), sibling parallel regions were incorrectly exiting and re-entering their entry actions.

Root Cause

getTransitionDomain had a guard !transition.reenter && that skipped the early-return path when reenter: true. This caused findLeastCommonAncestor([child, source]) to be invoked. Because isDescendant uses proper ancestor semantics, source is not considered a proper descendant of itself, so the LCA was computed as the parallel parent rather than source. With the parallel root as domain, all sibling regions were added to the exit and entry sets.

Fix

• getTransitionDomain: Remove the !transition.reenter guard so that when all targets are within the source, the domain is always the source (even with reenter: true).
• computeEntrySet: Explicitly add the source to statesToEnter when reenter && source === domain, because the ancestor-up-to-domain traversal excludes the domain boundary itself.
• SCXML tests: Skip more-parallel/test3b and test6b, which relied on the now-corrected domain computation to produce an unrelated conflict-resolution outcome. XState v5 already explicitly diverges from SCXML external-transition semantics (see the test506.txml comment in scxml.test.ts).

Verification

Failing case (reproduces #5162)

const machine = createMachine({
  type: 'parallel',
  states: {
    region1: {
      initial: 'a',
      states: { a: {}, b: {} },
      on: {
        REENTER: { target: '.a', reenter: true }
      }
    },
    region2: {
      entry: () => console.log('SHOULD NOT fire on REENTER'),
      initial: 'c',
      states: { c: {}, d: {} }
    }
  }
});

const actor = createActor(machine).start();
actor.send({ type: 'REENTER' });
// Before fix: region2 exits and re-enters (entry actions fire)
// After fix:  only region1 exits and re-enters ✓

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