fix(test-fill): Engine X fixtures never emit `postStateDiff`

## Problem

`BlockchainEngineXFixture` was designed to carry a `postStateDiff`: the accounts a test's execution changed relative to its pre-allocation group's genesis, as the format's small substitute for the full `postState` that `blockchain_test` and `blockchain_test_engine` fixtures carry. It has never been emitted: every Engine X fixture ever filled contains no post-state data at all (no `postState`, no `postStateDiff`, no `postStateHash`).

The wiring has been dead since the format was introduced in a404f8a16f1, hidden by three independent layers:

1. The filler computes the diff behind `hasattr(fixture, "post_state")`, but `BlockchainEngineXFixture` has no `post_state` field and its `extra: "ignore"` config silently drops the value that `make_hive_fixture` passes in, so `calculate_post_state_diff` is unreachable.
2. `post_state_validator` (which enforces that exactly one post-state field is provided, and would have raised on every Engine X fixture) never attaches its validator: the decorator defines it and then just does `return cls`.
3. Nothing on the consume side reads the field, so no consumer ever failed. Only the format docs describe it (`docs/running_tests/test_formats/blockchain_test_engine_x.md`).

Omitting the full `postState` from Engine X is deliberate (size, same reason `pre` is a `preHash` reference); omitting the diff is not: the field, the compute function, the docs, and the validator were all built for it.

## Impact

- The consume-side post-state verification the format docs describe is impossible.
- Fixing this enables the follow-up fill-time post-state drift check for pre-alloc group packing (#3122), which silently no-ops without it (#3161).

## Proposed fix

- Compute the diff in `make_hive_fixture` for the Engine X format, where the post-state `alloc` and the genesis baseline `pre` are both in scope (in phase 2, `pre` is the group's pre-allocation, so it is the correct diff baseline by construction); move `calculate_post_state_diff` out of the filler plugin and delete the dead post-processing block.
- Reimplement `post_state_validator` so it actually runs, with a test that it fires. Care: already-released Engine X fixtures legitimately contain neither field and must keep parsing on consume.