feat: concurrent mirror probing, download progress, and active version protection

- Replace serial mirror latency probing with concurrent probing using
  Zig 0.16 Io.Group fibers. Direct connections probe all mirrors in
  parallel (batch size 8), reducing total probe time from sum of all
  latencies to the slowest single probe. Proxy/curl path remains
  sequential since std.process.run blocks the event loop.

- Add download progress indicator showing percentage, downloaded/total
  size, and transfer speed. Uses chunked stream loop with ~10 updates
  per second. Clears line with ANSI escape to prevent ghost characters
  from shorter previous updates. Curl proxy path uses -# flag for
  native progress bar.

- Block removal of the currently active version with actionable
  guidance: list other installed versions for switching, or suggest
  installing a new version first when none are available.

Author: lispking