CC-2208: Upgrade Zig to 0.16

[andy1li]

Note

Medium Risk
Medium risk because this upgrades the Zig toolchain and updates process/IO/HTTP usage to new 0.16 APIs, which can affect build/run behavior in CI and user environments.

Overview
Upgrades the Zig track to Zig 0.16 across starters and solutions (required version, build.zig.zon minimum version, and codecrafters.yml buildpack), and adds a new dockerfiles/zig-0.16.Dockerfile for the updated build environment.

Updates build/run scripts to rely on the default zig build output (zig-out/bin/main) instead of a custom --prefix install path.

Refactors src/main.zig to the Zig 0.16 entrypoint style (main(init: std.process.Init)), switching argument/env handling and IO types (std.Io, injected io for std.http.Client and stdout streaming writes) to match the new standard library APIs.

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[linear-code]

CC-2208