docs + stress tests: domains publish reference, docstrings, 13 stress/E2E tests

- docs/reference/domains.md: new reference page covering muse domains dashboard, --new scaffold, publish command, capabilities manifest, auth, HTTP semantics, agent usage, plugin architecture, and end-to-end examples - muse/cli/commands/domains.py: expanded docstrings on _DimensionDef, _Capabilities, _PublishPayload, _PublishResponse, and _post_json (full Args/Returns/Raises) - tests/test_stress_domains_publish.py: 13 new stress and E2E tests covering 500-call throughput baseline, large capabilities (100 dims), unicode description, repeated 409 does not leak state, large --description (4000 chars), slug with hyphens, hub URL propagation, --json round-trip, --version semver, thread-safety (10 threads), full payload structure, capabilities auto-derived from MIDI plugin, 400-call CLI stress

mypy + typing_audit --max-any 0 + 3196 pytest tests all green