[Breadcrumbs] Build separators in one pass - #49058

What changed

• Builds the interleaved breadcrumb/separator array with push in one pass instead of repeatedly copying the growing accumulator with concat.
• Preserves item order, separator order, keys, and rendered output.
• No public API or observable behavior changes.

The previous reduction is quadratic in the number of children because each separator copies all preceding entries. The new reduction is linear.

Verification

• Synthetic 10,000-item, 7-sample benchmark:
  • before median: 271.284 ms
  • after median: 0.285 ms
• Exact output length, first-item, and last-item assertions passed.
• pnpm test:unit run Breadcrumbs --project 'node:@mui/material'
• pnpm -F @mui/material typescript
• Targeted ESLint and Prettier checks.
• Full pnpm release:build.
• Full Material Node unit suite: 179 passed / 4 skipped files; 4,712 passed / 759 skipped tests.

The browser test project was not run locally because Playwright Chromium is not installed in this checkout.

Contribution

• I have followed the contributing guide.

This change and PR description were prepared with OpenAI Codex, then self-reviewed and verified locally. No independent human review has occurred yet.