0.47.0 — GPS and self-noise gating for walk mode

Walk mode learns places and honesty about its own feet.

• ambiscape walk --gps track.gpx: GPX tracks give each zone a distance,
  mean speed and midpoint position (an independent cross-check on the
  acoustic step cadence), plus a route map coloured by zone.
• Self-noise gating: footfall and wind masks feed a gated per-zone level —
  what the place sounds like without the walker. On the Blindern reference
  walk the walking group was 2–4 dB of its own quiet zones.
• New guide page: Soundwalks (walk mode).

See CHANGELOG.md for details.