feat: support concurrent browser async context

[kasperpeulen]

Summary

• adds an unctx-powered compile-time async context transform for browser awaits
• routes Next app-render request/action/render async storage through the browser ALS shim using nested .run() calls instead of enterWith
• keeps ReadableStream lifecycles and nested async helper awaits inside the correct request frame
• adds concurrent ALS coverage, Next request-context coverage, and non-default browser API ports for playground suites
• pre-optimizes unctx in the Next playgrounds so browser-mode CI does not reload tests mid-import

Validation

• pnpm --dir packages/vitest-plugin-rsc exec vitest run src/async-local-storage.test.ts src/async-local-storage-transform.test.ts
• pnpm --dir packages/vitest-plugin-rsc run build
• pnpm tsgo --build
• pnpm format:check
• pnpm lint
• VITEST_BROWSER_API_PORT=64123 pnpm --dir playground/rsc-vitest-demo exec vitest run
• VITEST_BROWSER_API_PORT=64124 pnpm --dir playground/nextjs-notes-demo exec vitest run
• VITEST_BROWSER_API_PORT=64125 pnpm --dir playground/nextjs-no-msw-demo exec vitest run

[pkg-pr-new]

pnpm add https://pkg.pr.new/vitest-plugin-rsc@39 -D

commit: b97f472