Add reproducible release tooling foundation

[LunaStev]

Summary

• add a Python 3.11+ x.py entry point for release checks, builds, packaging, cleanup, and target discovery
• produce normalized tar.gz/zip archives with project documents, executable smoke checks, and portable SHA256SUMS
• test release-tool behavior across the Linux, Windows, and macOS CI matrix and add a native Linux package job
• document the release-tool contract while retaining the Makefile during migration

Why

Vex v0.0.1 needs one reproducible release path shared by maintainers and CI. The existing Makefile only covers Linux x86_64 and does not validate archive contents, versions, checksums, or official release preconditions.

Behavior

• python3 x.py check runs formatting, release-tool tests, Rust tests, Clippy, and a locked build.
• build and package default to the rustc host or accept explicit supported targets.
• packages contain the executable, README, LICENSE, NOTICE, and COPYRIGHT with normalized metadata.
• release requires a clean tree and the exact v<version> tag.
• checksums include only archives generated and verified by the current invocation.
• the Makefile remains for a later parity/removal PR.

Validation

• python3 x.py check
• python3 -m py_compile x.py tests/xpy/test_release_tool.py
• python3 -m unittest discover -s tests/xpy -v (13 tests)
• native Linux release build, package, executable smoke, and checksum verification
• repeated packaging with fixed SOURCE_DATE_EPOCH produced the same SHA-256
• workflow YAML parse and git diff --check

A local RISC-V attempt reached the selected cross linker but this host lacks the RISC-V glibc development sysroot. Existing Ubuntu RISC-V CI retains its cross-build and QEMU smoke coverage.