feat: add Nx shadow build system alongside fluid-build

[tylerbutler]

Background

We have long talked about moving off of fluid-build onto a more standard build system. Thanks to many changes to fluid-build over the years, it's easier now to set up a shadow build using Nx, so I did. This enables using Nx locally for cross-build caching, while still using fluid-build as the main build system. If Nx goes well, then we can pursue it. If it goes poorly, then we can remove it without much hassle.

Observations

• Nx is slower than fluid-build.
• Nx seems to run a lot more tasks when fully cached than fluid-build.
• The task dependencies are not quite right. They're good enough to get going, but they'll need tweaking to be fully correct.

Summary of changes

• Adds Nx as a shadow build orchestrator running alongside the existing fluid-build system across the client and build-tools release groups
• Introduces nx.json configuration at both the repo root (client) and build-tools/ with task dependency graphs, named inputs, cache settings, and target defaults that mirror the existing fluid-build task graph
• Adds 149 project.json files across all packages to override orchestrator tasks (e.g. build, compile, lint) with nx:noop, preventing Nx from invoking fluid-build scripts while letting executor tasks (e.g. tsc, eslint, test:mocha) run their real commands
• Includes per-package dependency overrides for packages with custom fluidBuild.tasks (e.g. client-utils, presence, tree, sequence, test-end-to-end-tests) to ensure Nx task prerequisites match fluid-build behavior
• Adds nx:* convenience scripts to root package.json files for running Nx commands (nx:build, nx:tsc, nx:affected:build, nx:graph, etc.)
• Adds nx and @nx/js/@nx/workspace as dev dependencies; updates pnpm lockfiles
• Documents the migration approach in docs/nx-migration-plan.md and build-tools/plans/nx-shadow-build-guide.md
• Adds .nx/ to .gitignore and .nxignore for cache and build-tools exclusions

[github-actions]

🔗 No broken links found! ✅

Your attention to detail is admirable.

linkcheck output

> fluid-framework-docs-site@0.0.0 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"


> fluid-framework-docs-site@0.0.0 serve
> docusaurus serve --no-open

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

> fluid-framework-docs-site@0.0.0 check-links
> linkcheck http://localhost:3000 --skip-file skipped-urls.txt

Crawling...

Stats:
  272202 links
    1863 destination URLs
    2108 URLs ignored
       0 warnings
       0 errors