[Elements2720]

Summary

• recognize SSR s-sn attributes while collecting non-shadow slotted nodes
• preserve nested default-slot content when the wrapper hydrates before its owning host
• ignore serialized slot-fb fallback nodes owned by another host
• preserve explicit slot attributes over serialized slot metadata
• add regression coverage for all three hydration boundaries

Fixes #6934

Tests

• npm run build -- --ci
• Runtime Jest suites: 52 suites, 357 tests passed
• Focused hydration suites: 3 suites, 20 tests passed
• Targeted ESLint and Prettier checks passed
• git diff --check passed

The full Jest suite has one unrelated existing failure in src/compiler/output-targets/dist-lazy/test/generate-lazy-module.spec.ts; repository-wide lint also reports pre-existing errors outside this change.