test: harden WordPress boot contracts for plugin/admin paths

[foo-bender]

Summary

• enforce explicit WordPress API contracts for plugin boot, plugin activation, and admin boot paths
• fail fast with actionable RuntimeException diagnostics when required APIs are missing
• add contract tests that verify stubs satisfy boot contracts and that missing APIs fail with explicit messages
• document boot-path WordPress stub coverage in docs/wp-stub-contracts.md

Validation

• composer test ✅ (85 tests, 409 assertions)
• composer lint ✅
• JS lint not run: no JS files changed, and CI runs npm run lint:changed:js only when JS changes are present

Fixes #7