nvx.py: retire the sandbox and run commands in favor of nvx-rs #725

Related: #722.

Goal

Callers run sandboxes and microVMs with nvx-rs directly, instead of through nvx.py sandbox and nvx.py run, so that nvx.py no longer fronts the runtime CLI, and nvx-rs no longer reproduces nvx.py's program name and argparse's layout.

Today

• nvx.py sandbox runs nvx-rs --prog PROGRAM --root ROOT sandbox ... with its standard streams, and exits with its status (scripts/nvx_tools/nvx_rs.py). nvx-rs: make the nvx-rs CLI the backend of nvx.py run #638 makes nvx.py run delegate to nvx-rs run the same way.
• nvx.py runs the nvx-rs that NVX_RS names, the one that a release ships as bin/nvx-rs, or one that it builds with Cargo (doc/usage.md).
• nvx::host::cli::args emulates argparse, so that usage and help match nvx.py's, wrapped at 80 columns, with the program name that --prog passes.
• doc/usage.md, doc/run.md, and the scenarios of nvx.py test-microvm run nvx.py sandbox and nvx.py run.

Tasks

• Decide how users of a checkout get nvx-rs: with Cargo, or with a command of nvx.py.
• Move the documentation, the examples, and the test-microvm scenarios to nvx-rs.
• Decide where the CPU-profile guidance goes. nvx-rs: make the nvx-rs CLI the backend of nvx.py run #638 has nvx.py run pass it to nvx-rs run to print after a failed cold boot, because its CPU catalog, in time_abi.py, also serves doctor and benchmark.
• Deprecate nvx.py sandbox and nvx.py run for a period during which they print the nvx-rs command to use, then remove them, and the parts of scripts/nvx_tools/nvx_rs.py that only they use.
• Remove --prog and the emulation of argparse's layout from nvx::host::cli::args, keeping usage errors with status 2.

Not in scope

• benchmark, doctor, the build commands, and the orchestration of test-microvm, which stay in Python (Track consolidating NVX host and guest code in the nvx-rs crate #356).

Acceptance criteria

• The documentation and the test suites run sandboxes and microVMs with nvx-rs.
• nvx.py has no sandbox or run command.
• nvx-rs has no --prog option, and no test pins argparse's layout.