fix(core): parallel top-level transitions preserve sibling region sub-state

[JSap0914]

Summary

Fixes #5214

Bug

When a transition is defined directly on a parallel state (its on property) and targets a specific sub-region, all other parallel regions were being reset to their initial sub-states.

const machine = createMachine({
  id: 'parallelExample',
  type: 'parallel',
  on: {
    ARCHIVE: { target: '#parallelExample.phase.archive' },
    EDIT: { target: '#parallelExample.mode.edit' },
  },
  states: {
    phase: { initial: 'inquiry', states: { inquiry: {}, archive: {} } },
    mode: { initial: 'new', states: { new: {}, edit: {} } },
  },
});

const actor = createActor(machine).start();
actor.send({ type: 'EDIT' });
// { phase: 'inquiry', mode: 'edit' }

actor.send({ type: 'ARCHIVE' });
// Before fix: { phase: 'archive', mode: 'new' }  ← mode wrongly reset
// After fix:  { phase: 'archive', mode: 'edit' } ← mode preserved ✓

Root Cause

In computeExitSet, when the transition domain is a parallel state, ALL active descendants of that domain were added to the exit set — including states in unrelated sibling regions.

In computeEntrySet, the domain (parallel state) was pushed into the ancestor list, which caused addAncestorStatesToEnter to call addDescendantStatesToEnter for every child of the parallel state not already targeted, resetting them to their initial sub-states.

Fix

When the transition's source IS the parallel domain and no target is the domain itself:

1. computeExitSet: only adds states in the parallel regions that contain the transition targets to the exit set. Sibling regions are left untouched.
2. computeEntrySet: skips pushing the parallel domain into ancestors, so addAncestorStatesToEnter never resets sibling regions.

Cross-region transitions (source is a child state, not the domain) and full-reset transitions that target the domain node itself (e.g. RESET: '#machineId') continue to work with the original behaviour.

Verification

pnpm --filter xstate test
# Test Files  73 passed (73)
# Tests       1728 passed | 13 skipped | 1 todo (1742)

New regression test: "transitions defined on a parallel state should not reset sibling regions to their initial sub-state" in packages/core/test/parallel.test.ts.

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