Save/load/navigate hooks to replace monkey-patching Story methods

[rohal12]

Problem

Host applications that need to inject behavior into Story.save(), Story.load(), and Story.goto() currently monkey-patch these methods:

const originalSave = Story.save.bind(Story);
Story.save = (slot, custom) => {
  Story.set('engine', serializeEngineState());
  originalSave(slot, custom);
};

This is fragile — on restart, each boot() wraps the methods again, creating a growing chain of closures. The host can't cleanly restore the originals without tracking them manually.

Proposed solution

Add event hooks that fire before/after these operations:

Story.on('beforesave', (slot, custom) => {
  Story.set('engine', serializeEngineState());
});

Story.on('afterload', (slot) => {
  restoreEngineState(Story.get('engine'));
});

Combined with #127 (auto-clean runtime handlers on restart), these hooks would be automatically cleaned up, eliminating the need to monkey-patch or manually track cleanup.

Suggested hooks

Hook	Fires	Use case
beforesave	Before save writes to storage	Inject engine state into story variables
aftersave	After save completes	Confirmation, analytics
beforeload	Before load restores variables	Cleanup current state
afterload	After load restores variables	Rebuild engine from restored state

Context

RoidRage wraps Story.save to serialize its engine snapshot into $engine before Spindle writes the save, and wraps Story.load/Story.goto for similar reasons. Making boot() idempotent requires either restoring original methods on teardown (manual tracking) or using proper hooks (clean, auto-cleaned by #127).