Promote minimal SharedTree branch APIs to beta

[Abe27342]

Description

This PR promotes the minimal set of APIs necessary to enable local branching flows in SharedTree.

[Copilot]

Pull Request Overview

This PR promotes the minimal set of SharedTree branch APIs from alpha to beta stability, enabling local branching workflows while maintaining backward compatibility through interface extension.

Key Changes:

• Split TreeBranch interface into beta (minimal) and alpha (extended) versions
• Created TreeViewBeta interface to expose beta-level branching capabilities
• Added asBeta() helper function for beta API access

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/dds/tree/src/simple-tree/api/tree.ts	Refactored TreeBranch into beta base interface and TreeBranchAlpha extension; created TreeViewBeta interface
packages/dds/tree/src/simple-tree/index.ts	Exported new TreeViewBeta and TreeBranchAlpha types
packages/dds/tree/src/simple-tree/api/index.ts	Re-exported new beta types
packages/dds/tree/src/shared-tree/treeAlpha.ts	Updated TreeAlpha.branch() return type to TreeBranchAlpha
packages/dds/tree/src/shared-tree/schematizingTreeView.ts	Updated method signatures to use TreeBranchAlpha
packages/dds/tree/src/api.ts	Added asBeta() function for beta API access
packages/dds/tree/src/index.ts	Exported TreeViewBeta, TreeBranchAlpha, and asBeta
packages/dds/tree/api-report/*.api.md	Updated API surface files with new beta exports

[Abe27342]

I have explicitly avoided promoting all of the alpha portions of TreeViewAlpha / TreeBranch into beta here, as some of these APIs may need more stabilization. I think the remaining set is still somewhat usable.

[Josmithr]

Nit: I think elsewhere we have generally documented more restricted API variants more like:

/**
 * {@link TreeBranch} with alpha-level APIs
 */

Alternatively, if the docs are identical, we could leverage an {@inheritDoc} comment.

Either way, seems like we should avoid duplicating docs when possible.

[Josmithr]

Left one nitpick, otherwise looks good to me!

Though it might be worth adding a very quick changeset noting which APIs were promoted, and that they can now be imported via /beta (for completeness).

[github-actions]

🔗 Found some broken links! 💔

Run a link check locally to find them. See
https://github.com/microsoft/FluidFramework/wiki/Checking-for-broken-links-in-the-documentation for more information.

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

 ELIFECYCLE  Command failed with exit code 1.