TypeScript 6 upgrade for PR 4887

Goal

Make the current checkout pass with TypeScript 6, or identify the exact blocker if the ecosystem is not ready.

Context

• Root package.json currently pins typescript to 5.8.3.
• The repo already has documented pitfalls around mixed src/dist graphs and misleading filtered Turbo typecheck failures.
• The user supplied the TS 6 migration guide. I still need primary-source confirmation for toolchain compatibility before changing versions.

Relevant learnings

Workspace alias split-brain

• Source: docs/solutions/developer-experience/2026-03-12-typescript-workspace-subpath-aliases-in-apps-www.md
• Takeaway: do not trust noisy type explosions until I confirm whether the TS program is mixing package src and dist.

Turbo filtered typecheck can lie

• Source: docs/solutions/test-failures/2026-03-24-turbo-filtered-typecheck-can-lie-when-package-typecheck-passes.md
• Takeaway: if filtered Turbo typecheck fails, verify the package directly and rerun serialized before declaring real debt.

Plan

1. Confirm TS 6 release + compatibility of key tooling from primary sources.
2. Reproduce the failing check on this checkout.
3. Upgrade the minimum required deps/config for TS 6.
4. Fix real breakage exposed by the upgrade.
5. Run install/build/typecheck/lint verification in the required order.
6. Decide whether this work produces reusable knowledge and capture it if yes.

Verification gate

• pnpm install
• pnpm turbo build ...
• pnpm turbo typecheck ...
• pnpm lint:fix

Errors encountered

• None yet.