feat: add optimistic JSON Patch rebase lab

[developer-1px]

Summary

• add a private patch-rebase lab that plans conservative delayed local changes over ordered concurrent JSON Patch batches
• shift local operation targets and one headless selection across concrete array insertions/removals while keeping numeric object keys distinct
• reject overlapping, root, move/copy, and other unsupported cases as structured conflicts instead of silently overwriting data
• prepend value guards so the plan remains atomic across the planning-to-commit gap, then revalidate the combined schema result
• canonicalize ordinary and URI-fragment JSON Pointers before path arithmetic, including encoded delimiters and root aliases
• keep the core public API unchanged and document CRDT, transport, causal ordering, stable identity, and ABA detection as non-goals

Validation

• npm run verify in labs/extensions/patch-rebase (18 passed)
• npm run labs:extensions:verify (39/39 verified)
• npm run standard:check
• npm run docs:check
• npm run docs:evaluate
• package build and self-import
• independent blocker review of transform, guard, selection, and URI-fragment behavior
• git diff --check