codeSplitters: build an x86 box's Rust crates in the hosted deploy container #373

Follow-up from #351. Hosted deploys now refuse a Hetzner box whose native/ crates would have to compile (the config read in apps/codesplitters/src/preview.ts, CONFIG_READ), saying to deploy it from the desktop.

For x86 boxes (cpx*, cx*, ccx*) that could instead work from the site: the deploy container is already linux/amd64, so a Rust toolchain in deploy-sandbox/Dockerfile would let rustybuns build box compile them natively. The CLI would need to build them when the target is the host (today it only compiles cross targets, in Docker, and otherwise expects native/dist). Costs: a bigger image and a slower build inside the 13-minute run budget. cax boxes would still need Docker or an arm64 runner.

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