Ship shared vitest/tsconfig presets — start's examples carry ~262 lines of identical config copies

[btravers]

Found while auditing btravstack/start (2026-08-13).

Measured across start's 11 example workspaces: ~262 lines of config files are byte-identical copies —

• src/vitest.d.ts (import type {} from "@unthrown/vitest";): 11 identical copies
• vitest.config.ts (the 9-line node/include/setupFiles/coverage block): 9 identical copies
• tsconfig.test-d.json (6 lines): 9 identical copies
• tsconfig.json: one 9-line "library" variant × 6, one 13-line "app" variant × 4
• the same 3-script block (test / test:types / typecheck) in 9 of 11 package.json files

This repo already ships @btravstack/tsconfig; the same move covers the rest. Every btravstack monorepo with more than a couple of workspaces pays this copy tax, and drift between copies is silent (start's two divergent vitest.config.ts are legitimate, but nothing distinguishes a deliberate divergence from a stale copy).

Acceptance

• A shared vitest preset (defineConfig-mergeable or an extendable base) covering environment, include, setupFiles: ["@unthrown/vitest"] and the coverage defaults, so a workspace's vitest.config.ts states only its deltas.
• A published tsconfig.test-d.json base, and the library/app tsconfig variants if they are not already expressible from @btravstack/tsconfig.
• start's examples adopt them and delete the copies.