[External integration] Use third-party survey files directly (umbrella)

[vpicaver]

Umbrella issue for treating third-party survey-tool files as the working source instead of one-shot importing them. CaveWhere should be able to open a Compass / Walls / Survex project, work with it in place. CaveWhere will not modify these files or write changes back in the native format.

Scope (child issues)

• [Compass][External integration] Use .mak and .dat files directly #508 — Compass .mak / .dat direct support
• [Walls][External integration] Use Walls project/survey files directly #509 — Walls .wpj / .srv direct support
• [Survex][External integration] Use Survex .svx files directly #510 — Survex .svx direct support

Supporting work

• Update survex submodule to v1.4.21 #519 — Update survex submodule to v1.4.21 (prerequisite for trusting cavern as the in-process resolver)

Cross-cutting questions

These need a shared answer across all three children so the UX is consistent:

1. Cave / trip mapping. Each format breaks the data up differently (Compass files, Walls books, Survex *begin blocks). CaveWhere's model is cwCavingRegion → cwCave → cwTrip → cwSurveyChunk. We need one convention per format that round-trips cleanly.
2. Survex as the reference engine. We feed Compass and Walls data into Survex for loop closure (Survex ships readers for both). Survex's interpretation of each upstream format is therefore the ground truth for what direct-mode CaveWhere must preserve. Investigate Survex's behavior for each format up front, before locking in the Compass/Walls direct designs.
3. Mixed editing. Decide whether users can still author survey data inside CaveWhere when a project is backed by an external format, or whether the external files are the sole source of truth in that mode.
4. Persistence model. How this interacts with .cw / .cwproj (Git-backed) projects — is the external file tree referenced by the CaveWhere project, or does CaveWhere open the external project directly with no .cwproj wrapper?

Related (not a child)

• [External Integration][Pocket Topo]Add support for importing centerline data for PocketTopo #503 — PocketTopo centerline import. Different tool, import-style rather than direct, but worth keeping in mind as we shape the import-vs-direct boundary.