feat(stellar): add wraith names on-behalf flow

[sudo-robi]

closes #10

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[truthixify]

The on-behalf work is solid — register_on_behalf / update_on_behalf / release_on_behalf are all there, the authorization message helper centralizes the canonical-bytes definition, replay protection is tested (test_register_on_behalf_replay), and the property-based roundtrip is exactly what we need. The proptest snapshot files committed under test_snapshots/ are the right call for CI determinism — keep them.

The blocker is the base — this PR was opened against an older main and is now conflicting hard with develop. The current diff would undo recently-merged work (the proptest harness in #13, the event-topic design doc in #26, the announcement-schema audit in #27, the stuck-funds rescue tool in #28).

Could you rebase onto develop?

git fetch origin
git rebase origin/develop
# don't delete: EVENT_TOPIC_DESIGN.md, ANNOUNCEMENT_SCHEMA.md, POSTMORTEMS.md,
#               scripts/rescue-stealth-funds.ts, scripts/tests/, scripts/README.md,
#               scripts/package.json/.lock, stellar/{announcer,registry,sender}/tests/properties.rs,
#               the stellar-nightly CI job
# your snapshot files under stellar/wraith-names/test_snapshots/ are fine to keep as new files
git push --force-with-lease

Once that's rebased I'll re-review and merge. Really nice work on the auth message + replay nonce design — thanks @sudo-robi.