Say which order the k of a silent payment output follows (#768)

[fametrano]

btclib/psbt/silent_payments.py's module docstring described BIP375's
prose -- the codes sharing a scan key sorted lexicographically by spend
key -- and named _ordered_sp_outputs as that sort. The function is
output index order, and its own docstring says so and says why, so the
file contradicted itself and the published API docs carried the half that
is not the code.

The function's own account gains the correction the discrepancy needed:
the two invalid vectors named after ordering are refused under either rule
because each carries a k assignment no ascending rule produces (two
values swapped in one, three permuted in the other), not because they
match none at all -- measured by deriving every candidate k for each
output with BIP375's own compute_silent_payment_output_script. It also
names bips PR 2207, which
would settle the question the other way by correcting the vectors and the
validator to the prose; against those vectors the test that pins index
order is what fails.

No behaviour change. Gates run locally: uv run pytest (27808 passed),
uv run pre-commit run --files ..., and the sphinx -W build.

Issue #768

Summary by Sourcery

Clarify documentation around silent payment output ordering to match the implemented index-order behavior and explicitly document the discrepancy with BIP375 prose and test vectors.

Enhancements:

• Refine _ordered_sp_outputs documentation to accurately describe why invalid BIP375 test vectors are rejected under any ascending rule and reference bips PR 2207 as the proposed resolution.

Documentation:

• Update psbt.silent_payments module and function docstrings to state that k counts outputs in index order per scan key and to explain the mismatch with BIP375 prose, reference vectors, and validator.

[sourcery-ai]

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[sourcery-ai]

Reviewer's Guide

Clarifies and documents that the silent payments k-ordering implementation uses output index order (matching BIP375 test vectors and reference validator) rather than the prose-described lexicographic spend-key order, and updates comments/changelog to explain how invalid vectors are rejected and to reference the relevant BIPs PR.

File-Level Changes

Change	Details	Files
Updated silent_payments module and helper docstrings to accurately describe k-ordering as output index order and to explain the discrepancy with BIP375 prose and test vectors.	Adjusted the module-level docstring in psbt.silent_payments to state that k counts per scan key over outputs in index order, not lexicographically by spend key. Clarified that _ordered_sp_outputs documents the discrepancy between BIP375 prose and its vectors/validator, and that specific tests pin the behavior. Extended the _ordered_sp_outputs docstring to describe how invalid vectors are rejected under any ascending rule, detailing the swapped/permuted k assignments and referencing bips PR 2207.	btclib/psbt/silent_payments.py
Documented the k-ordering clarification and behavior in the project changelog.	Added a changelog entry explaining the previous contradiction between the module docstring and implementation for k-ordering in psbt.silent_payments. Documented that the module docstring now aligns with the implementation and points to _ordered_sp_outputs for the BIP375 discrepancy. Noted how invalid vectors are handled under both ordering rules and that bips PR 2207 may change the BIP, with tests guarding current behavior.	CHANGELOG.md

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