Rebase Vite/Vitest tooling onto deps-update stack

[OBrezhniev]

Rebase tooling migration onto our deps/prover stack (Vite + Vitest)

Applies the feat/esm-migration build/test tooling on top of our
feature/deps-update work instead of replaying it, so nothing we shipped
is lost. Target: merge this into feat/esm-migration.

• Rollup → Vite (build/main.cjs unchanged in shape), Mocha → Vitest,
  flat eslint.config.js (with vitest globals), CI workflow.
• Keeps our git+https commit-ref pin for ffjavascript and our audit
  overrides (serialize-javascript / diff). Local dev uses an uncommitted
  file:../ffjavascript override.

Validation: lint clean, 3 tests pass, CJS require target verified.

[OBrezhniev]

ESM-rebase set — the tooling migration re-applied on top of our optimization stack (rather than replaying the branches), so the MSM/CIOS/memory work is preserved. Review together:

• Rebase ESM + Vite/Vitest onto CIOS/MSM stack wasmcurves#76 (full CJS→ESM; keeps our CIOS/GLV hot files)
• Rebase Vite/Vitest tooling onto MSM stack (build/test only, no threadman rewrite) ffjavascript#186 (Vite/Vitest only — WorkerSlot/batch-MSM threadman kept, no workerpool)
• Rebase Vite/Vitest tooling onto direct-io stack fastfile#137 · Rebase Vite/Vitest tooling onto our stack (keep MAX_BUFFER_SIZE fix) binfileutils#89 · Rebase Vite/Vitest tooling onto deps-update stack #106
• Rebase Vite/Vitest tooling onto prover stack (self-contained IIFE, external bfj) snarkjs#629 (self-contained IIFE + external-lazy bfj kept)

Deviations from feat/esm-migration are called out in each PR body (e.g. binfileutils MAX_BUFFER_SIZE, snarkjs IIFE/bfj, mnt6753 excluded, smart_contracts test deferred). All branches keep git+https commit-ref pins; re-pin if a branch moves.