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

Cover shared idle tracker behavior:
- tracker lifecycle (new, idle after timeout, touch resets)
- read/write with data touches tracker
- read retries on timeout when tracker is not idle
- read closes on timeout when tracker is idle
- shared tracker prevents false timeout across directions