fix(receive): Enforce BIP-352 K_MAX per-group recipient limit in scan_txouts

[MusabYK]

Description

Fixes #67 by adding const K_MAX: u32 = 2323 to silentpayments::receive, and a threshold check to the matched_tweaks counter loop in scan_txouts. Also added a ScanLimitExceeded(u32) variant to SpReceiveError returning an explicit error

Notes to the reviewers

I replaced check_cases(27) with a dedicated
maximum_per_group_recipient_limit_k_max_is_exceeded() test function that explicitly matches and asserts SpReceiveError::ScanLimitExceeded(K_MAX) on test vector 27, preserving check_cases for standard test vectors. i think modifying it with hardcoded branch conditions for test 27 makes the general test runner dirty.

All Submissions:

• I've signed all my commits
• I followed the conventional commit guidelines
• I ran just p (fmt, clippy and test) before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[nymius]

Can you squash both commits together? Also, in general, I prefer commit messages that explain the what and why of the code, rather than short descriptions.

[MusabYK]

Hi @nymius I squashed both commits together and updated the commit message to include the full context. Thanks for the feedback

[nymius]

One last request, can you wrap the commit message to less than 72 characters?

[MusabYK]

One last request, can you wrap the commit message to less than 72 characters?

Done!

[nymius]

ACK 1598b06

[nymius]

If you want to work in a follow up: I prefer these slow tests to only execute on CI, and ignore them in just pre-push.

[MusabYK]

Makes sense. I'll look into it.