fix: broken ESM entry, colliding net ids, daemonize pidfile races

[AlexanderSlaa]

Summary

Four bugs found together while using this library to daemonize 3-NIC,
2-virtio-drive VMs from a real provisioning script (nested-VM cluster PoC):

• exports["."].import pointed at dist/index.es, an extension Node's
  ESM loader doesn't recognize -- plain import "node-qemu" fails with
  ERR_UNKNOWN_FILE_EXTENSION. The package was unusable via import as
  published on npm (1.2.1), only via require. Renamed the ESM build
  output to dist/index.mjs.
• Every net entry without an explicit id defaulted to the literal
  "net0" regardless of position (unlike drives, which already
  indexes its default id as drive${i}), so 2+ net entries collided
  with Duplicate ID 'net0' for netdev.
• A virtio: true drive never got if=none on its -drive clause, so
  QEMU's legacy "no if= means if=ide" auto-attach fought the explicit
  -device virtio-blk-pci attach: Drive 'driveN' is already in use ... did you need 'if=none'?.
• With daemonize: true, the spawned process forks and exits almost
  immediately as part of normal, successful daemonization -- but the
  exit handler treated that exit like the real process dying and
  deleted config.pidfile, racing against (and usually winning against)
  QEMU's own -pidfile write for the actual detached daemon. Both the
  manager's own pre-write and the delete-on-exit are now skipped when
  daemonize is set; QEMU owns that file exclusively in that mode.

Test plan

• npx vitest run -- 101/102 pass; the 1 remaining failure
  (tests/image/qemu-img.test.ts, a require() of a .ts path)
  pre-exists on main unrelated to this change, confirmed via
  git stash.
• Added regression tests for all 3 args.ts/manager.ts fixes.
• npx vite build then node -e "import('./dist/index.mjs')..." --
  resolves and exposes all expected exports.
• End-to-end: launched a real 3-NIC, 2-virtio-drive, daemonized
  qemu-system-x86_64 VM with the patched build; boots, all 3 taps
  come up with correct MACs, SSH reachable, pidfile stable after the
  launcher process exits.

Note: package-lock.json is untouched -- installing locally with npm
9.2.0 (this sandbox's version) rewrote unrelated libc metadata across
the lockfile; reverted that noise before committing.

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