[codex] Add rrweb-snapshot utility boundaries

[Juice10]

Summary

• add snapshot-domain and rebuild-domain utility entrypoints in rrweb-snapshot
• route snapshot and rebuild internals through those entrypoints without changing imported symbols
• keep the legacy utils export public and document the compatibility path

Validation

• yarn workspace rrweb-snapshot check-types
• yarn workspace @rrweb/record build
• yarn workspace @rrweb/record test
• rg -n "postcss" packages/record/dist --glob '*.{js,cjs}' (no matches)

[changeset-bot]

⚠️ No Changeset found

Latest commit: c9afb6b

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[Juice10]

Superseded by origin-hosted stacked PR rrweb-io#1838 so checks run against rrweb-io workflows.