chore: clean up stale changesets — content shipped via release-it

The Changesets - Version PR workflow has been failing on `node-liblzma`
because the root package isn't included in pnpm-workspace.yaml's
`packages: ['packages/*']` glob. The `native-memlimit.md` changeset
referenced `node-liblzma` and triggered "Found changeset for package
not in the workspace" errors on every master push.

Architectural reality: this monorepo uses release.yml (release-it +
conventional commits) as the canonical release path. Changesets
artifacts are decorative — never consumed by the actual release
flow. Their content is already captured in CHANGELOG.md via
release-it's conventional-commit scan.

Cleanup:
- .changeset/native-memlimit.md (PR #112, node-liblzma not in
  workspace, blocked the workflow)
- .changeset/wasm-memlimit-option.md (PR #111, same reason)
- .changeset/validate-changesets-flow.md (test file from
  changesets adoption — no longer needed)
- .changeset/tar-xz-streaming.md (PR #113, shipped via
  tar-xz@6.1.0 release-it 2026-04-29)
- .changeset/win32-toctou-hardening.md (PR #114, shipped via
  same tar-xz@6.1.0 release)

Keep .changeset/config.json + README.md for the directory's
identity. Future PRs can still add changesets if the changesets
workflow is later re-enabled as a primary path.

Unblocks: changesets.yml workflow on master pushes.

Author: oorabona