Fix example statechart modeling

[SandroMaglione]

Summary

• redesign the microwave, media-player, and Pokémon examples so invalid domain configurations are structurally unrepresentable
• reserve parallel states for genuinely independent regions and use declared initial states for default entry
• keep control-only states schema-less while preserving named tagged unions/classes for data-bearing states
• keep the media player's live HTML audio element behind its Effect service boundary so machine snapshots remain free of process-local resources
• document statechart topology, schema ownership, .initial, and appropriate matches usage for users and agents

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• Relevant example checks, when examples changed
• Automated type-performance measurement passed or was not required
• Automated runtime- and memory-performance measurement passed or was not required

Additional validation:

• examples/playground: pnpm check — 8 tests, typecheck, and production build
• examples/pokemon: pnpm check — typecheck and production build
• examples/platformer: pnpm check — 4 tests, typecheck, and production build
• browser verification of the media-player mount and snapshot display with no console errors