feat: add optional collaboration providers

[developer-1px]

Summary

• add a transport-free causal collaboration provider that preserves the six-member JSONDocument API used by the local provider
• keep selective history and collaborative text authoring in opt-in subpaths, with DOM/IME publication leasing in a separate companion package
• preserve the dependency-free Core and exact archive boundary; local-only consumers install neither companion
• extend CI, package checks, and the explicit active-package catalog without restoring archived 1.x extensions

Verification

• Core verify: 185 passed, 4 skipped
• Collaboration verify: 104 passed
• Contenteditable collaboration verify: 17 passed
• v2 standardization, archive isolation, docs evaluation, site build/tests/smoke, and 3 browser tests passed
• Core and both companion tarballs passed dry-run packaging
• Fresh consumer install passed JavaScript runtime and TypeScript root/history/text/DOM entrypoint smoke checks