chore: backport 12 PRs to v4-next (#20512, #20379, #20831, #21024, #21134, #21072, #21186, #21189, #21229, #21228, #21234, #21639)

[AztecBot]

Summary

Combined backport of 12 PRs to v4-next, cherry-picked in chronological merge order:

1. docs: add delayedpublicmutable apiref, fix misc docs #20512 — docs: add delayedpublicmutable apiref, fix misc docs
2. feat: custom message handlers in Aztec.nr #20379 — feat: custom message handlers in Aztec.nr
3. feat!: make unused msg disco fns private #20831 — feat!: make unused msg disco fns private
4. 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
5. chore: add warning on invalid recipients #21134 — chore: add warning on invalid recipients
6. feat: add aztecaddress::is_valid #21072 — feat: add aztecaddress::is_valid
7. chore: use returns true for boolean fns #21186 — chore: use returns true for boolean fns
8. feat: add note hash and nullifier helper functions with domain separation #21189 — feat: add note hash and nullifier helper functions with domain separation
9. docs: small delayedpubmut update #21229 — docs: small delayedpubmut update
10. test: restore pubmut tests #21228 — test: restore pubmut tests
11. fix: claim contract & improve nullif docs #21234 — fix: claim contract & improve nullif docs
12. feat!: split compute note hash and nullifier to reduce hashing #21639 — feat!: split compute note hash and nullifier to reduce hashing

Each PR is a separate commit (raw cherry-pick with conflicts left in), followed by a single final commit that resolves all conflicts — making it easy to review the conflict resolution independently.

Conflict resolution (last commit)

• delayed_public_mutable.nr / public_immutable.nr — merged doc comments from both sides
• aztec.nr macros — integrated custom message handler + split compute functions
• discovery/mod.nr / process_message.nr — merged offchain inbox sync + custom handler dispatch + split compute
• constants.nr / constants_tests.nr — added new domain separators
• Nargo.toml — added custom_message_contract
• netlify.toml — merged error code redirects
• auth_contract — kept v4-next delay value
• note_metadata.nr — doc comment reformat
• migration_notes.md — merged migration notes from both sides
• Deleted traits.nr (v4-next uses traits/ directory)

ClaudeBox log: https://claudebox.work/s/3145d1bd30977c20?run=1

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/5a1e314ad3ac09dc�5a1e314ad3ac09dc8;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_epochs/epochs_mbps.parallel.test.ts "builds multiple blocks per slot with transactions anchored to checkpointed block" (177s) (code: 0) group:e2e-p2p-epoch-flakes