test(validator): add validator sync-lag duty gate test vectors

[zemse]

🗒️ Description

Introduces a new consensus fixture format for the validator sync-lag duty gate and lands the first vectors.

validator_duty_gate format

• Replays a sequence of steps against ValidatorService._is_synced_for_duties and records the allow or skip verdict per step. The gate stays the source of truth, the fixture only records what it decides.
• Each step gives a wall-clock slot and an optional head move. The hysteresis flag carries across steps, so one vector can drive the gate both open and closed.

Initial vectors (2)

Under tests/consensus/lstar/validator/, two step sequences cover the decision surface:

• Threshold and hysteresis: lag 4 allowed and lag 5 gated (open threshold), lag 3 staying closed while lag 2 reopens (hysteresis band pinned to 2), and a head ahead of the wall clock saturating lag to 0.
• Network-stall escape: net-lag 8 still gates while net-lag 9 escapes (strict boundary), the escape reopening a closed gate and keeping an open one signing, for both block and attestation duties.

The missing-head fallback and run-loop wiring are left out as client-internal behavior, covered by each client's own unit tests.

🔗 Related Issues or PRs

Mirrors the validator sync-lag duty gate introduced in #708.

✅ Checklist

• Ran local quality checks to avoid unnecessary CI fails:

  just check

• Considered adding appropriate tests for the changes.
• N/A for docs, test-vector and fixture-format-only PR, the format is documented inline.