test(sdk): add coverage for #195-landed cross-society types

[dp-web4]

Summary

Completes integration wiring + test coverage for the cross-society MCP types
that landed on main via PR #195 but lacked tests, error registry entries, and
package exports. Re-scoped per PR #196 rejection directive.

• errors.py: CrossSocietyError class (was in __all__ but undefined), 6 ErrorMeta
  registry entries for §7.6 cross-society codes, _CATEGORY_SUBCLASS dispatch
• init.py: Export 6 cross-society MCP types + CrossSocietyError (369→376 exports)
• test_mcp_cross_society.py (NEW, 37 tests): enum values, construction, defaults,
  to_dict/from_dict round-trips, frozen checks, package-level import verification
• test_errors.py: Updated assertions (24→30 codes, 6→7 categories), cross-society
  subclass/metadata/serialization tests

No mcp.py formatting churn. No version bump. No CLI changes.

Quality gates

• 2746 passed, 8 xfailed (conformance gaps, pre-existing)
• mypy --strict: 0 errors across 26 source files
• ruff check: 0 errors
• ruff format --check (changed files): 0 changes
• GitNexus detect_changes: risk level LOW, 0 affected processes

Test plan

• python -m pytest tests/test_errors.py tests/test_mcp_cross_society.py -v — 87 passed
• python -m pytest tests/ -q — full suite 2746 passed, 8 xfailed
• python -m mypy --strict web4/ — clean
• python -m ruff check web4/ tests/ — clean
• Package exports verified: import web4; len(web4.__all__) == 376

🤖 Generated with Claude Code

[dp-web4]

APPROVED: Fixes a real defect on main — CrossSocietyError is exported in web4/errors.py all (origin/main line 36) but the class was never defined, an import/AttributeError hazard for any consumer touching that name. This PR defines the class, adds the 6 §7.6 cross-society ErrorMeta registry entries + _CATEGORY_SUBCLASS dispatch, exports the 6 cross-society MCP types (369→376, confirmed by CI 'Exports: 376'), and adds 37 new tests (test_mcp_cross_society.py) plus correctly updates test_errors.py assertions (24→30 codes, 6→7 categories). Diff matches description exactly; re-scoped per the prior #196 rejection directive (reviewer already engaged this work). CI: 2742 passed, 8 xfailed (documented pre-existing). The only 4 failures (test_cli version, test_package_api version) are a systemic CI-infra problem — the package reports 'web4 0.0.0' because the version isn't injected during the CI editable install — and reproduce identically on every web4 PR regardless of content. The lint 'Would reformat: web4/mcp.py' is pre-existing unformatted code on main (this PR does not touch mcp.py), and the wheel-filename error is a CI workflow glob bug — all systemic, none introduced here. Not bottlenecking a correct, well-tested bug fix on repo-wide infra noise; the version-injection / mcp.py-format / wheel-glob issues should be filed as a separate CI-hardening task.