feat(consume): add consume enginex simulator

[danceratopz]

🗒️ Description

• WIP: needs test/verification, especially fcu-frequency & xdist & client stop/clean-up functionality
• WIP: needs doc
• Requires refactor(fill,fixtures): rename "engine reorg" to "engine x"; improve pre-allocation group terminology #1760.
• Requires hive branch from WIP: Enable shared clients on testsuite level hive#1285

Adds a simulator that runs BlockchainEngineXFixture against clients.

Running The Simulator & Initial results

I've been testing locally with a subset of 1846 tests that create 29 groups from Cancun:

uv run fill --output=fixtures --clean -x -m "not zkevm and not slow" tests/cancun --fork=Cancun --evm-bin=../evmone/build/bin/evmone-t8n -n 8 --generate-pre-alloc-groups

Then consume against a hive dev server:

uv run consume enginex --input=fixtures-cancun --durations=5 --dist=loadgroup -n 8 --enginex-fcu-frequency=0

Results for 1846 tests with 29 groups (=^ 29 client initializations):

• reth: 85.37s (0:01:25).
• besu: 361.71s (0:06:01).
  (client versions at Pectra Fork).

FCU Behavior

# Disable all FCUs (fastest execution)
uv run consume enginex --enginex-fcu-frequency=0 fixtures/

# FCU every test (current behavior)  
uv run consume enginex --enginex-fcu-frequency=1 fixtures/

# FCU every 3rd test per pre-allocation group
uv run consume enginex --enginex-fcu-frequency=3 fixtures/

Xdist Behavior

Tests get distributed to xdist worker by pre-allocation group using loadgroup:

uv run consume enginex --input=fixtures-cancun --durations=5 --dist=loadgroup -n 8 --enginex-fcu-frequency=0

🔗 Related Issues

• feat(fill): enable shared pre-allocation groups and add BlockchainEngineReorgFixture #1706
• refactor(fill,fixtures): rename "engine reorg" to "engine x"; improve pre-allocation group terminology #1760

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.