feat: add opt-in v2 hydration marker support

[Jane Chu (janechu)]

Pull Request

📖 Description

Adds coordinated, opt-in support for FAST Element 2.x indexed hydration markers while retaining FAST Element 3.x data-free markers as the default.

• Exports markers_v2 from @microsoft/fast-element/hydration.js for use with enableHydration({ markers: markers_v2 }).
• Adds a matching markers_v2 option to @microsoft/fast-build across its Rust, WASM, CLI, and configuration APIs.
• Isolates v2 marker parsing from the default v3 implementation.
• Adds a FAST Build-generated declarative fixture that exercises normal hydration, repeat reconciliation, references, attributes, and events with v2 markers.
• Updates migration guidance, package documentation, design documentation, and export sizes.

👩‍💻 Reviewer Notes

The v2 compatibility path is explicitly opt-in on both sides:

enableHydration({ markers: markers_v2 });

{
    "markers_v2": true
}

The client and renderer settings must match. Calls without these options continue using FAST Element 3.x data-free hydration markers.

📑 Test Plan

• Build and test the microsoft-fast-build Rust crate.
• Build and test @microsoft/fast-build, including CLI/config option forwarding.
• Build @microsoft/fast-element.
• Run the FAST Element Chromium and declarative Chromium suites.
• Verify the generated v2 fixture hydrates and supports repeat updates, attribute bindings, references, and events.

✅ 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.

Agents

• I have linked to an existing issue in this project that this change addresses
• I have read the skills
• I have read the DESIGN.md file(s) in packages relevant to my changes
• I have updated the DESIGN.md file(s) in packages relevant to my changes