docs: explain what mapping.txt is and how to retrace a release crash #193

docs/testing.md has only a short note on mapping.txt (#183). Explain it properly:

• What it is: R8 renames classes, fields and methods in release builds (com.openipc.decoder.DecodePlanner → a.u, capacity → a) and keeps line numbers only through the map. A crash from a release reads like at a.u.a(SourceFile:12) until it is retraced.
• What is in the file: the R8 version, min_api and pg_map_id (a hash that ties it to one build) in the header, then original -> obfuscated for every class, field and method with line ranges, including library classes. It holds no secrets (the code is open), and devices don't need it.
• Which file to use: the mapping of the same build. That is decoder-v2-mapping.txt in latest for the current build, or for an older build the …-mapping artifact of the CI run of that commit (find the run by the hash shown under Exit or in /stats.json; artifacts expire after the repository's retention period). This depends on Releases: keep only latest and test pre-releases, no per-version releases #194.
• How: retrace, with an example, before and after.