fix(testing): make equivocation head assertions scheme-independent

[leolara]

Problem

The Production Test Vectors job failed on main (run 28858797181): 3 fork-choice equivocation vectors failed, 551 passed:

test_same_slot_equivocating_attesters_count_once            head_slot = 2, expected 3
test_equivocation_head_independent_of_arrival_order_a_then_b head_slot = 3, expected 2
test_equivocation_head_independent_of_arrival_order_b_then_a head_slot = 3, expected 2

Root cause

PR #1181 breaks an equal-slot equivocation tie toward the fork with the largest attestation-data root. That root embeds the target block root, which embeds the genesis state root, which embeds the validator XMSS public keys — so the winner flips between the test and prod schemes. The 3 vectors hardcoded the winning fork, so they passed under test (what PR CI runs) and failed under prod (what prod-vectors runs, on main only). Same class of bug as #916.

Fix

The vectors now assert the invariant, not the lottery winner:

• New scheme-independent check canonical_equivocation_head_among: reads each fork's attestation root from the store and asserts the head is the largest-root fork. Mirrors the existing lexicographic_head_among; never pins which fork wins.
• Rewrote the 3 vectors to use it, dropping the scheme-fragile head_slot/head_root_label and the equivocators' target_slot pins. Given/When/Then docstrings describe the invariant.
• Docstring on the tiebreak (fork_choice.py) flags that the winner's identity is key-material-dependent.
• CLAUDE.md: new rule — fork-choice head assertions must be scheme-independent.

No consensus-rule change; the tiebreak behavior is unchanged.

Testing

• The 3 vectors pass under both --scheme=test and --scheme=prod.
• Full fork-choice tree (123 vectors) passes under both schemes.
• just check clean.

[tcoratger]

@leolara can we reduce the number of line for the new documentation as per doc writer agent rules to keep things lean and facilitate the reading experience ? :)

[leolara]

@leolara can we reduce the number of line for the new documentation as per doc writer agent rules to keep things lean and facilitate the reading experience ? :)

You are right, we are all learning about how to use these new tools.