feat(server): expose session-lifetime baseline RTT to the embedder

[Greg Lamberson (glamberson)]

Summary

• RttSnapshot.min_ms is a sliding-window low that can rise as low samples
  age out of the window, and is explicitly documented as not baseRTT per
  MS-RDPBCGR 2.2.14.1.5, which defines baseRTT as the session-lifetime
  lowest. AutoDetectManager already tracks that true low internally as
  min_rtt_ms for the wire NetworkCharacteristicsResult, but nothing
  exposed it as its own value: an embedder reading snapshot() or
  autodetect_rtt_handle() alone cannot derive averageRTT - baseRTT as a
  queueing-delay signal, since that relationship only holds when the
  floor cannot rise.
• Adds AutoDetectManager::baseline_rtt_ms() as the public getter for the
  existing field, and mirrors the autodetect_rtt_handle plumbing on
  RdpServer: a new autodetect_baseline_rtt: Arc field,
  autodetect_baseline_rtt_handle() accessor, and
  with_autodetect_baseline_rtt_handle() builder method.
• A matched RTT sample always updates min_rtt_ms in the same
  handle_response call that returns it, so the store site reads the new
  getter unconditionally right after storing the RTT sample, with no
  before/after comparison needed (unlike the bandwidth case in feat(server): expose measured bandwidth to the embedder #1734,
  where the underlying value can be cleared to None on an unusable
  measurement).
• Adds a manager-level test pinning the session-low-not-window-low
  property on the new getter directly, plus the usual pair of
  handle-plumbing tests (sentinel default, injected-handle round trip).

Validation

cargo xtask check fmt/lints/tests/typos/locks all pass, including the
3 new tests.

Notes

No public API break: RdpServer::new is crate-private, and the public
surface (RdpServerBuilder) only gains an additive optional field and a
new method.