docs: specify limb range assumptions for KZG point evaluation precompile

[ivokub]

Description

Documents an input precondition of the EIP-4844 point evaluation gadgets in
std/evmprecompiles (KzgPointEvaluation, KzgPointEvaluation16,
KzgPointEvaluationFailure, KzgPointEvaluationFailure16).

Context. The gadgets take versionedHash, commitmentCompressed and
proofCompressed as native limbs of 16 bytes (128-bit variant) or 2 bytes
(16-bit variant). Each limb is decomposed with conversion.NativeToBytes and
only the low bytesPerLimb bytes are used. The discarded high bytes are not
constrained to be zero, so a limb v + k·2^(8·bytesPerLimb) is treated
the same as v. If the limbs are not range checked, multiple public limb
values map to the same precompile input.

Why documentation instead of an in-circuit check. These gadgets are meant
to be called from the Linea arithmetization, where the input limbs are already
range checked. Asserting the range again would add constraints for a check
already done by the caller. Instead, the method documentation now states
explicitly that the caller must range check every limb to 128 bits
(respectively 16 bits), and that the method does not check this.

Changes:

• KzgPointEvaluation / KzgPointEvaluation16: add a paragraph describing the
  limb range assumption and what happens when it does not hold.
• KzgPointEvaluationFailure / KzgPointEvaluationFailure16: point to the
  above for the range assumption, alongside the existing pointer for the data
  encoding.

Scope notes:

• expectedBlobSize and expectedBlsModulus are compared for equality with
  constants and are not affected.
• evaluationPoint and claimedValue are emulated elements and are not
  affected.

Type of change

• This change requires a documentation update

How has this been tested?

• Documentation-only change; go vet ./std/evmprecompiles and gofmt are clean.

How has this been benchmarked?

N/A — no circuit changes.

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• I did not modify files generated from templates
• golangci-lint does not output errors locally

🤖 Generated with Claude Code

---

Note

Low Risk
Comments-only change with no logic or constraint changes; it clarifies an existing caller obligation for correct circuit soundness.

Overview
Documentation-only update for EIP-4844 KZG point-evaluation gadgets in std/evmprecompiles.

KzgPointEvaluation and KzgPointEvaluation16 now document that callers must range-check every limb of versionedHash, commitmentCompressed, and proofCompressed (128-bit vs 16-bit per variant). The gadgets only take the low bytes from each limb via NativeToBytes and do not enforce that high bits are zero, so unconstrained limbs do not uniquely bind the precompile input.

KzgPointEvaluationFailure and KzgPointEvaluationFailure16 doc comments now point readers to those same encoding and range assumptions. No circuit or runtime behavior changes.

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[yelhousni]

accurate and complete