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

The hardcoded noise range (2500-4700 bytes) in the FakeTLS ServerHello
does not match the real certificate chain sizes of many popular fronting
domains (e.g., dl.google.com ≈ 6480 bytes, microsoft.com ≈ 13004 bytes).
This makes the proxy detectable by DPI systems that compare the
ApplicationData size with the real cert chain size for the SNI domain.

On startup, probe the fronting domain's actual TLS handshake size and
use the measured value ± jitter instead of the static range. Falls back
to the legacy 2500-4700 range if the probe fails.

Also adds optional caching of probe results between restarts
(noise-cache-path, noise-cache-ttl) and a configurable probe count
(noise-probe-count) under [defense.doppelganger].

Closes #408