z: add test, verify commands and Nanvix OS download step

[ppenna]

Extend the build utility with end-to-end testing and verification capabilities for the Nanvix cross-toolchain.

Changes

download_steps now fetches the Nanvix OS multi-process release (libposix.a, user.ld, and nanvixd binaries) via gh release download when not already present in the install location.

New commands:

• ./z test — runs a smoke test that cross-compiles .nanvix/tests/hello.c with the installed ${TARGET}-gcc, then (if KVM is available) executes the resulting ELF on nanvixd.elf and asserts value = 42 in the output.
• ./z verify — checks that all expected toolchain artifacts (compiler binaries, libraries, Nanvix OS files) are present after installation.

CI pipeline updated to enable KVM, run verify, and run the smoke test after the stage-1 build completes.

Files

• z — new functions: test_smoke_c, test_integration_c, test_steps, verify_steps; extended download_steps
• .nanvix/tests/hello.c — minimal C test program
• .github/workflows/ci.yml — added verify and test steps with KVM enablement

[Copilot]

Pull request overview

This PR extends the ./z build utility with post-install validation: it can now download required Nanvix OS runtime artifacts, verify that an installed cross-toolchain has the expected files, and run a basic end-to-end C compile (and optionally execute the result under nanvixd when KVM is available). CI is updated to run the new verification and smoke test steps after the stage-1 install.

Changes:

• Extend download_steps to fetch Nanvix OS release artifacts (libposix.a, user.ld, nanvixd binaries) via gh release download.
• Add ./z verify (artifact presence checks) and ./z test (compile smoke test + optional KVM integration run).
• Update GitHub Actions workflow to enable KVM and run verify/test after installation.

Show a summary per file

File	Description
z	Adds Nanvix OS download logic plus new test/verify commands and supporting functions.
.nanvix/tests/hello.c	Adds a minimal C program used by the smoke/integration tests.
.github/workflows/ci.yml	Exports GH_TOKEN, enables KVM permissions, and runs ./z verify and ./z test post-install.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 4

[Copilot]

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 3