Rebase Vite/Vitest tooling onto direct-io stack

[OBrezhniev]

Rebase tooling migration onto our direct-io stack (Vite + Vitest)

Applies feat/esm-migration tooling on top of our
feature/direct_rw_optimization work. Target: feat/esm-migration.

• Rollup → Vite (build/node/main.cjs + browser esm/iife), Mocha/Chai → Vitest
  (node-esm / node-cjs / browser projects), flat eslint.config.js, CI.
• Adopts your explicit browser/node split (src/fastfile.browser.js +
  exports.browser), which supersedes our runtime-detection + constants.browser.js
  compat approach. Our direct-io fast paths in osfile.js are untouched and
  still used by the node fastfile.js (type:file → osfile.open()); our
  BigBuffer direct-io regression tests are ported into the Vitest osfile suite.
• Keeps our audit overrides (serialize-javascript / diff / minimatch).

Validation: 20 node + 8 browser tests pass; CJS require + browser ESM/IIFE 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 #137 · Rebase Vite/Vitest tooling onto our stack (keep MAX_BUFFER_SIZE fix) binfileutils#89 · Rebase Vite/Vitest tooling onto deps-update stack r1csfile#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.