chore: backport 11 PRs to v4-next (docs, aztec-nr, domain separation)

[AztecBot]

Summary

Combined backport of 11 PRs to v4-next, cherry-picked in order with a final conflict resolution commit.

Included PRs (in order):

1. docs: add delayedpublicmutable apiref, fix misc docs #20512 — docs: add delayedpublicmutable apiref, fix misc docs
2. feat!: make unused msg disco fns private #20831 — feat!: make unused msg disco fns private
3. feat: add compile-time size check for events and error code links #21024 — feat: add compile-time size check for events and error code links
4. chore: add warning on invalid recipients #21134 — chore: add warning on invalid recipients
5. feat: add aztecaddress::is_valid #21072 — feat: add aztecaddress::is_valid
6. chore: use returns true for boolean fns #21186 — chore: use returns true for boolean fns
7. feat: add note hash and nullifier helper functions with domain separation #21189 — feat: add note hash and nullifier helper functions with domain separation
8. docs: small delayedpubmut update #21229 — docs: small delayedpubmut update
9. test: restore pubmut tests #21228 — test: restore pubmut tests
10. fix: claim contract & improve nullif docs #21234 — fix: claim contract & improve nullif docs
11. feat!: split compute note hash and nullifier to reduce hashing #21639 — feat!: split compute note hash and nullifier to reduce hashing

Structure

• Commits 1-11: Raw cherry-picks (may contain conflict markers)
• Commit 12: Resolves all conflicts

Conflict resolution notes

• traits.nr was deleted on v4-next-staging — kept deleted
• migration_notes.md — merged additively
• aztec.nr / discovery/mod.nr / process_message.nr / test_environment.nr — adapted split compute functions to v4-next-staging codebase
• constants.nr — merged new domain separators with existing ones
• delayed_public_mutable.nr — merged API reference docs with updated descriptions

ClaudeBox log: https://claudebox.work/s/baa724fa6390303f?run=1

[nventuro]

Closed in favor of #21745.