poc(data-migration): Add migration machinery to FluidDataStoreContext which swaps out entire factories

[markfields]

Description

Copilot's summary of the change after we chatted about it a bit:

Implement one-hop, load-time datastore migration during first realization only. Old factory exposes migrationInfo (with portableData + newPackagePath). Context detects it, lets the old runtime catch up then gets its portable data and disposes it without binding. Next it swaps pkg, loads new factory via an extended migratable factory interface, instantiates new runtime with instantiateForMigration, binds it, and and proceeds normally. Summaries then carry new package path.

Breaking Changes

Some short-term back compat will be needed across the datastore / runtime boundary.

Reviewer Guidance

Proof of concept, plenty of gaps/todo's.

[github-actions]

🔗 No broken links found! ✅

Your attention to detail is admirable.

linkcheck output

> [email protected] ci:check-links /home/runner/work/FluidFramework/FluidFramework/docs
> start-server-and-test "npm run serve -- --no-open" 3000 check-links

1: starting server using command "npm run serve -- --no-open"
and when url "[ 'http://127.0.0.1:3000' ]" is responding with HTTP status code 200
running tests using command "npm run check-links"


> [email protected] serve
> docusaurus serve --no-open

[SUCCESS] Serving "build" directory at: http://localhost:3000/

> [email protected] check-links
> linkcheck http://localhost:3000 --skip-file skipped-urls.txt

Crawling...

Stats:
  235713 links
    1720 destination URLs
    1953 URLs ignored
       0 warnings
       0 errors

[markfields]

This is "Approach 3" in a document circulating internally. It's very future-looking, not relevant for short-terms plans.