perf: reduce hydration target traversal overhead

[Jane Chu (janechu)]

Pull Request

📖 Description

Hydration target discovery currently checks every TreeWalker candidate against a DOM Range. This change replaces that per-node Range membership work with a bounded traversal and uses the live Range only when synchronous custom-element reactions mutate the traversal boundaries.

• Preserves existing hydration marker behavior, mismatch diagnostics, and mutation compatibility.
• Does not change public APIs, marker formats, or default hydration behavior.
• Removes redundant content-node snapshots and traversal state.
• Reduces every affected minified bundle by 129–144 bytes compared with main; Brotli output changes by +40–79 bytes.

👩‍💻 Reviewer Notes

Please focus on the boundary-mutation fallback in target-builder.ts. The normal path performs zero NodeFilter callbacks and zero Range.comparePoint() calls for 107 traversal candidates; the mutation fixture uses two live-Range checks and is bounded to five in the covered scenarios.

📑 Test Plan

• 63 focused target-builder tests across Chromium, Firefox, and WebKit.
• 319 Chromium hydration and repeat tests.
• 199 Chromium declarative tests.
• FAST Element TypeScript build, Biome/lint, aggregate package build, 34 export checks, API Extractor checks, Beachball validation, and git diff --check.

✅ 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

[Copilot]

Pull request overview

Optimizes FAST Element hydration target discovery while preserving marker and mutation behavior.

Changes:

• Adds bounded TreeWalker traversal with live-Range mutation fallback.
• Adds comprehensive cross-browser traversal tests.
• Updates bundle sizes and release metadata.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/fast-element/test/main.ts	Exposes the target builder to browser tests.
packages/fast-element/src/hydration/target-builder.ts	Implements optimized hydration traversal.
packages/fast-element/src/hydration/target-builder.pw.spec.ts	Covers boundaries, mutations, markers, and performance.
packages/fast-element/SIZES.md	Records updated bundle sizes.
change/@microsoft-fast-element-893a7afa-7056-4d39-9257-150cb9bf26f8.json	Adds the patch release note.

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