[codex] Fix factorial work distribution and update for Zig 0.16

[mjackson]

What changed

This PR fixes a real bug in the factorial implementation and updates the project to build cleanly with Zig 0.16.

Why it changed

The repo no longer built with the current Zig toolchain because both the build API and several stdlib APIs had changed.

While updating the code, I also found a separate bug in factorial.zig: the work distribution logic returned a slice backed by stack memory and also assumed a maximum of 8 threads even though the thread count is user-controlled.

Impact

• zig build now succeeds on Zig 0.16
• zig build test now succeeds on Zig 0.16
• factorial work distribution no longer uses invalid memory or overruns a fixed-size stack buffer when more than 8 threads are requested

Root cause

• The build script and command modules targeted older Zig APIs such as root_source_file, std.io.getStdOut, std.fs.cwd, and older process/random helpers.
• The factorial work range code constructed a slice from a local array and returned it, which made the caller read invalid memory after the function returned.

Validation

• zig build
• zig build test

[mjackson]

Superseded by #3 from the correctly prefixed branch.