Validate ephemeral-account lifecycle and sponsorship boundaries

@bzawisto is already working on this.

Since Sep 11, 2026.

## Outcome

Ephemeral-account creation, resizing, and closure must enforce caller authority and move sponsor/vault lamports correctly.

These operations have different rules from ordinary delegated accounts, and the current security scenarios do not cover their complete lifecycle.

## Scope

Through an authorized fixture program, create an ephemeral account, grow and shrink it, close it, and recreate it. Record account owner, size, lamports, and sponsor/vault balances before and after each operation.

Submit conflicting resize/close/recreate transactions against the same address. Add negative cases using a foreign caller, missing sponsor signature, substituted vault, occupied address, and prohibited direct top-level invocation.

Use an independent payer where needed so transaction fees do not obscure sponsor/refund accounting.

## Acceptance criteria

- [ ] Valid operations produce the expected account state and sponsor/vault balance changes.
- [ ] Invalid calls fail without unauthorized account or sponsor/vault changes, apart from separately accounted permitted transaction fees.
- [ ] Conflicting operations resolve to a valid serialized outcome without duplicate refunds.
- [ ] Closure and recreation do not retain unauthorized ownership or stale account contents.