Don't download VDatum grids during build — check and warn instead

[rolker]

Problem

The mru_transform CMakeLists.txt downloads VDatum grids (~1.7GB) and PROJ geoid data during cmake configure time. This makes builds:

• Slow — first build downloads ~1.7GB over the network
• Non-reproducible — depends on external server availability
• Offline-hostile — fails in air-gapped or field environments with spotty internet
• Surprising — unexpected disk usage and network activity during a colcon build

Proposed Solution

Change the CMake build to check and warn instead of downloading:

1. Check if grids exist in ~/.cache/mru_transform/ (or a configurable path)
2. If missing, message(WARNING "VDatum grids not found — run download_vdatum_grids.sh") and skip the install step
3. If present, install them into the share directory as today

The existing scripts/download_vdatum_grids.sh already handles the download — make it the documented way to pre-populate the cache. The chart_datum_node should log an error and stay inactive if grids are missing at runtime.

This treats grid data as a deployment step, not a build dependency — similar to ENC charts or firmware.

---

Authored-By: Claude Code Agent
Model: Claude Opus 4.6