v1.5.0a20

• GPS walk analysis tool (python -m vanchor.analysis.gps_walk). Feeds the
  no-RTK station-keeping design (#154): run it over a stationary debug
  recording and it reports the M9N's position-walk envelope (std/CEP50/CEP95),
  the walk's autocorrelation time, the averaging-window sweet spot, the Doppler
  velocity noise floor at rest, and whether the reported hAcc is honest --
  ending with suggested hold-controller constants (velocity deadband, outer
  position-loop tau). Reads .ndjson/.ndjson.gz parts, stdlib only.
• Faster boot, especially offline. Three changes: (1) the SD image no longer
  gates boot on "network online" -- NetworkManager-wait-online blocked Docker
  (and therefore vanchor) for its full 30-60 s timeout on the internet-less boat
  AP; (2) firmware delays trimmed (disable_splash=1, boot_delay=0); (3) the
  saved depth map/chart now loads in the background after the server is up --
  a big imported chart took seconds on a Pi before the UI would answer at all;
  overlays briefly render blank and pop in when loaded. Estimated ~30-90 s
  faster to a responsive UI on an offline boot (image changes need a reflash;
  the depth-load change arrives with the bundle).