bip-0375: say the version byte belongs to the identifier, not to the field

[fametrano]

One sentence, in the Unique Identification section:

The PSBT_OUT_SP_V0_INFO should be serialized as a zero byte for the
version, followed by the 33 bytes of the scan key and then 33 bytes for
the spend key.

It names the field, and the field is 66 bytes: its table entry is
<33 byte scan key> <33 byte spend key>, bip375_test_vectors.json carries
66, and validator/validate_psbt.py refuses anything else — invalid[1]
reports "Output 0 SP_V0_INFO has wrong length (65 bytes, expected 66)".

Read in its section the sentence is not about the field but about the output
script that stands in for a silent payment output when building the unsigned
transaction used for unique identification, where 67 bytes contradicts
nothing. That is also where it came from: d29e2f8 added it together with
the unique-identification paragraph above it.

Both readings are available to someone reading the sentence alone, and one of
them contradicts three other artefacts. This picks the one that does not,
without changing anything normative.

No changelog entry or version bump: nothing about the format changes, and
leaving them out keeps this from conflicting with
#2207 or
#2256, both of which touch the
changelog. Glad to add both if you would rather have them.

scripts/link-format-chk.sh, scripts/buildtable.pl,
scripts/diffcheck.sh and typos pass locally.

Noticed while implementing BIP375 in btclib, which serializes the version
byte in the identifier and not in the field:
btclib-org/btclib#768

[jonatack]

@fametrano Thank you for your contribution. Out of personal curiosity, what AI did you use to make 228 GitHub contributions (so far) today? The PR description is a little hard to grok, can you summarize more concisely in your own words, please?

[fametrano]

@jonatack in my own words: PSBT_OUT_SP_V0_INFO is 66 bytes everywhere else in the BIP (field table, vectors, validator), but one sentence in Unique Identification says to serialize it with a leading version byte, which is 67. That sentence describes the placeholder output script used to compute the identifier, not the field. The rewording makes that reading the only one available; nothing normative changes.

On the tooling question: Claude Code, driven by me. The measurements are run locally and I review every text before it is posted; most of that day's contributions were in my own btclib-org repositories, where BIP375 was being implemented. Point taken on the descriptions, I will keep them shorter.