Merge dev into v4-dev and migrate the split workspace to Heft

[AJIXuMuK]

Q	A
Bug fix?	[x]
New feature?	[x]
New sample?	[x]
Related issues?	N/A — integration of the existing dev branch

What's in this Pull Request?

Merge dev at 6ee720f into v4-dev at
0361654, preserving both histories in
merge commit 9b36396.

• Preserve the Rush-managed controls library and private test web part/customizer app, component identities, library 4.0.0 version line, SPFx peer-dependency boundaries, and workspace:* linking.
• Integrate dev fixes, improvements, new controls, public exports, localization, documentation, and samples into the split layout. The upstream file audit checked 1,378 source/test/script/typing/documentation files with no missing mapped files.
• Migrate both projects to Heft, SPFx 1.23.2, TypeScript 5.8.3, ESLint 9, and Node 22.14.0+ within the Node 22 release line. Consumers require SPFx 1.23.2+ below 2.0.0 and React 17.
• Preserve discovery of colocated tests and the separate library test directory; update legacy fixtures and the React 17 adapter without disabling diagnostics or tests.
• Replace Gulp-dependent fast-serve with native Heft serving, retaining the independent Sonar Gulp task. Nonfatal lint warnings remain visible; compiler, test, and packaging errors still fail builds.
• Update CI/release workflows and development documentation for the split workspace.
• Respect user-configured npm registries and generate a registry-neutral lockfile with integrity hashes retained.
• Exclude test/build infrastructure from the npm package while shipping public entrypoints, declarations, and localization assets.
• Add Rush change metadata without incrementing the v4 version line.

Validation used Node 22.22.0:

Command	Result
fnm exec --using=22.22.0 node common/scripts/install-run-rush.js install	Passed with the regenerated frozen lockfile.
fnm exec --using=22.22.0 node common/scripts/install-run-rush.js rebuild	Passed: 185 library tests, 1 app test, production bundles, and both .sppkg files. Lint warnings remain visible.
fnm exec --using=22.22.0 node --test common/tests/pnpmfile.test.cjs	Passed: 4 registry-portability tests. No mirror-specific URLs remain in the lockfile.
fnm exec --using=22.22.0 node common/scripts/install-run-rush.js change --verify --no-fetch --target-branch origin/v4-dev	Passed after committing the generated change file.
fnm exec --using=22.22.0 npm pack --dry-run --json in libraries/spfx-controls-react	Passed: required entrypoints, declarations, and localization present; test/build infrastructure excluded.
fnm exec --using=22.22.0 node_modules/.bin/heft start --nobrowser in apps/controls-tests	Passed: webpack compiled and /temp/build/manifests.js returned HTTP 200 with the app manifest. Server stopped afterward.

Both pinned branch tips are ancestors of the merge commit. No packages were
published and neither remote dev nor v4-dev was directly updated.
SharePoint-hosted interactive behavior and deployment still require tenant
validation. Rush 5.135.0 emits its Node 22 compatibility notice, and legacy
dependency/lint warnings remain.

Guidance

• You can delete this section when you are submitting the pull request.
• Please update this PR information accordingly. We'll use this as part of our release notes in monthly communications.
• Please target your PR to dev branch.
• This integration explicitly targets v4-dev at the user's request, rather than the template's general dev target.