[Breadcrumbs] Build separators in one pass

[OskarEichler]

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.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-49058--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	▼-5B(0.00%)	▼-9B(-0.01%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

---

Check out the code infra dashboard for more information about this PR.