[Code/Doc Hygiene] TOB-RIPCTXR Appendix C roundup (typos, dead reimpl, doc mismatches)

[tesseract-ripple]

Severity: Informational; trivial fixes.
Source: TOB May 2026 revision, Appendix C — "Non-Security-Related
Recommendations" (pp. 56-58). mpt-crypto-side items only;
C.1 (XLS-0096 README terminology sort) and C.2 (XLS-0096 README
typos) belong in XRPLF/XRPL-Standards, not this repo. C.3
(canonical encrypted zero counter retry) is tracked separately in
#108.

This issue bundles the five remaining mpt-crypto items from
Appendix C into one cleanup PR.

Items

1. C.4 — Extraneous z^2 in BP verifier comment.
   src/bulletproof_aggregated.c around L2629-L2630 (current line
   may differ post-MSM-refactor; locate by string). The comment
   reads:

   /* P += sum_{k=0}^{n-1} [ (-z)*G_k + ( z*y^k + z^(block+2)*z^2*2^i ) *
    *   (y^{-k}*H_k) ] */
   

   The intended exponent is z^(block+2)*2^i, without the extra
   z^2 factor.

2. C.5 — secp256k1_mpt_pedersen_commit reimplements
   mpt_uint64_to_scalar. src/commitments.c#L226-L231 does the
   uint64 → 32-byte BE scalar conversion manually instead of
   calling the shared helper at mpt_internal.h#L60-L67.

3. C.6 — Documentation typo. mpt_utility.h#L425 reads
   "Helper function to substract a transparent amount from a
   hidden commitment." Replace "substract" with "subtract".

4. C.7 — Documentation / implementation size mismatch.
   include/utility/mpt_utility.h#L44-L54:
   mpt_pedersen_proof_params documents pedersen_commitment as
   "64-byte" but the array dimension is kMPT_PEDERSEN_COMMIT_SIZE,
   which is 33. Update the comment to "33-byte (SEC1-compressed)".

5. C.8 — Stale parameter doc.
   include/utility/mpt_utility.h#L456-L472:
   mpt_verify_send_range_proof's doxygen lists a @param ctx
   that is not in the function signature. Either remove the
   @param ctx line or pass ctx through.

Acceptance

• All five fixes land in one PR.
• Pre-commit + clang-format clean.
• 11/11 ctest still green; no behavior change.

References

• TOB May 2026 Appendix C, pp. 57-58 (Figures C.5 through C.10).