docs(lstar): clarify verify_signatures and ban "# Why:" labels

[tcoratger]

Summary

Documentation-only cleanup of the lstar block signature verifier (verify_signatures), plus a small readability refactor of its body and one new documentation rule.

• Docstring rewrite — spell out the proof contract (the proof carries neither public keys nor messages, so the caller reconstructs both and the verifier checks against them) and show the two parallel lists as a layout diagram.
• Correct Raises: — the function raises SpecRejectionError with one of three concrete reasons (VALIDATOR_INDEX_OUT_OF_RANGE, PROPOSER_INDEX_OUT_OF_RANGE, INVALID_BLOCK_PROOF), not a bare AssertionError. Clients assert on .reason, so the docstring now names the real contract.
• Lean inline comments — each body step (key resolution, message binding, proposer entry, proof check) gets a glued one-line comment, one sentence per line.
• Small refactor — drop a redundant to_validator_indices() recomputation and rename the loop variables to voter_index / voter_indices to disambiguate from the proposer index. No behavior change.
• New doc rule — ban the # Why: inline-comment label. A comment only exists when the reason is non-obvious, so the label is redundant noise; state the reason directly as plain prose.

Testing

just check passes (ruff lint, ruff format, ty, codespell, mdformat). No behavior change, so no fixtures regenerated.

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