feat(lstar): add tiered block-production strategy as a selectable alternative

[MegaRedHand]

What

Adds a second proposer-side block-building algorithm for the lstar fork, kept alongside the existing one as an interchangeable mixin.

Mixin	File	Selection policy
BlockProductionMixin (default)	block_production.py	round-based fixed-point: re-scan until a pass adds nothing
TieredBlockProductionMixin (new)	block_production_tiered.py	tiered greedy scorer: finalize > justify > build, projected incrementally

Both share the build_block contract declared on LstarSpecBase, so a fork composes exactly one.

How to switch

The default stays the round-based selection. To select the tiered scorer, swap a single import in spec.py:

# from lean_spec.spec.forks.lstar.block_production import BlockProductionMixin
from lean_spec.spec.forks.lstar.block_production_tiered import (
    TieredBlockProductionMixin as BlockProductionMixin,
)

then regenerate:

uv run fill --fork=lstar --clean -n auto

Test changes: fixtures generate under either mixin

Two fork-choice fillers used to assert the simple builder's exact block body. The two builders include a different number of votes in those scenarios, so generation aborted when the tiered builder was wired in. Both fillers now generate cleanly under either mixin without any test depending on which mixin is active (verified: 540 vectors pass and the determinism check passes, under both).

Each filler is made mixin-independent in the way that fits its scenario:

• test_produce_block_enforces_max_attestations_data_limit — switched to a single-voter, justifiable-target scenario. No vote reaches the two-thirds supermajority, so no entry justifies its target and the entry cap alone bounds the body. Both builders therefore fill to exactly the limit, and the scenario asserts that count for both. (This is the more robust setup originally written for feat: tiered attestation scoring #791; it changes this one committed vector.)

• test_justified_divergence_self_heals_in_next_block — the divergence here is real and same-scenario: the fixed-point builder keeps a vote already recorded on-chain, the tiered builder drops it as adding no new voters. That body difference is a proposer-strategy choice, not a consensus outcome, so it is no longer asserted. The test instead asserts the justified slot, which is the strategy-independent witness of the self-heal: the chain can only reach justified slot 1 once the block incorporates the slot-1 votes.

The committed vectors are generated by the default (simple) strategy.

Notes

• just check passes (lint, format, ty, codespell, mdformat).
• just deadcode passes.
• Supersedes the replace-approach in feat: tiered attestation scoring #791.

[MegaRedHand]

Note: the dead-code CI check will be red on this PR until #1150 merges.

#1150 fixes a pre-existing stale vulture_whitelist.py on main (the hash_tree_root handlers were renamed _htr_* -> _hash_tree_root_* in #1124 without updating the whitelist). That fix has been kept out of this PR to keep it focused. Once #1150 lands on main, merging main here clears the dead-code failure; the only whitelist change this PR then carries is the TieredBlockProductionMixin entry.