feat: modularize FASTElement with opt-in hydration and subpath exports

[janechu]

Pull Request

📖 Description

Modularizes @microsoft/fast-element to reduce core bundle size and make hydration, styles, and array observation opt-in through subpath exports.

Hydration is now opt-in — enableHydration() from @microsoft/fast-element/hydration.js activates hydration via a pluggable hook. Without it, ElementController has zero hydration imports or logic.

New subpath exports split heavy features out of the main barrel:

• ./styles.js — css, ElementStyles, CSSDirective, style strategies
• ./arrays.js — ArrayObserver, Splice, SpliceStrategy, array utilities
• ./hydration.js — enableHydration, deferHydrationAttribute

globalThis.FAST removed — FAST is now a module-scoped export with no side-effecting global mutation. FAST.getById(), FASTGlobal, KernelServiceId, and the requestIdleCallback polyfill are all removed.

Per-element lifecycle callbacks are passed directly to declarativeTemplate(), and isPrerendered / isHydrated are now separate promises with clear semantics.

👩‍💻 Reviewer Notes

• TemplateElement.config() is preserved as a deprecated shim for backward compatibility.
• ElementController now uses composition for PropertyChangeNotifier instead of inheritance.
• The hydration rendering path (renderPrerendered) moved entirely into enable-hydration.ts via ElementController.installHydrationHook().
• isPrerendered resolves true when a DSD shadow root existed (independent of hydration). isHydrated resolves true only when hydration actually ran.
• Declarative template {{ }} bindings with spaces inside braces do not work — the templates.html files must use {{binding}} without spaces.

📑 Test Plan

• 1396 Playwright tests pass (chromium)
• 176 declarative fixture tests pass (chromium)
• New declarative-no-hydration fixture (5 tests) verifies client-side rendering, lifecycle callbacks without hydration, and isPrerendered/isHydrated semantics
• Updated platform.pw.spec.ts and debug.pw.spec.ts for module-scoped FAST
• Updated lifecycle-callbacks and performance-metrics fixtures for new API

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

⏭ Next Steps

• Run full CI across all browsers (Firefox, WebKit)
• Remove TemplateElement.config() deprecated shim in a future release
• Consider lazy-loading the Compiler on first render for further size reduction

[janechu]

Deep review pass focused on the fast-element modularization, hydration opt-in path, packaging, and docs.

[janechu]

Re-review follow-up: the original hydration runtime/config, standalone styles, dts rollup, subpath API extraction, and website migration table fixes are mostly addressed. I found a few remaining API/doc edge cases. One non-inline note because the file is not changed in this PR diff: packages/fast-element/MIGRATION.md still points HydrationControllerCallbacks to ElementHydrationCallbacks via ElementController.configHydration() and still says the install-hydration import has no replacement; that should be updated to the new enableHydration() / HydrationOptions migration path.

[janechu]

Latest re-review: the previously resolved threads mostly check out, but I found a few remaining follow-ups. Non-inline note because this file is still not in the PR diff: packages/fast-element/MIGRATION.md still points HydrationControllerCallbacks to ElementHydrationCallbacks via ElementController.configHydration() and still says @microsoft/fast-element/install-hydration.js has no replacement; that guidance should be updated to the new enableHydration() / HydrationOptions migration path.

[janechu]

Latest re-review: the hydration TSDoc, repeated enableHydration() registration, and styles/arrays API report exports are mostly fixed. Non-inline note because the file still is not part of the PR diff: packages/fast-element/MIGRATION.md still points HydrationControllerCallbacks to ElementHydrationCallbacks via ElementController.configHydration() and still says @microsoft/fast-element/install-hydration.js has no replacement; that guidance should be updated to the new enableHydration() / HydrationOptions migration path.

[janechu]

Latest re-review: the hydration callback chaining fix checks out and regression checks passed. One non-inline note remains because the file is still not part of this PR diff: packages/fast-element/MIGRATION.md still points HydrationControllerCallbacks to ElementHydrationCallbacks via ElementController.configHydration() and says @microsoft/fast-element/install-hydration.js has no replacement; that guidance should be updated to the new enableHydration() / HydrationOptions migration path.