go install github.com/dolonet/mtg-multi@latest was broken because go.mod
declared module path as github.com/9seconds/mtg/v2 while the repo lives
at github.com/dolonet/mtg-multi. Users ended up installing upstream
binary without multi-secret support.

Fixes https://github.com/9seconds/mtg/issues/376#issuecomment-4162877568

1. Add sync.Mutex to ScoutConnCollected to eliminate data race between
   Add()/MarkWrite() in readLoop and learn() iterating results.
   Introduce Snapshot() for safe read access.

2. Increase bloom filter test size from 500 to 100000 to prevent
   false negatives from random eviction in the stable bloom filter.

3. Use Require().NoError() in TestHTTPSRequest to prevent nil-pointer
   panic on resp.Body.Close() when the request fails.

Fixes #425