NetworkBytesDecoder

Platform-specific wire-format decoder used by CachedElevationSourceFactory.fetchTile on cache miss. Each platform implements the same MIME dispatch table (BIL16 / BIL32 / TIFF / DTED → ElevationTileBuffer) — sharing the dispatch in commonMain would mean porting every per-platform decoder kernel here, which buys little vs. the per-platform footprint cost.