fix(build): compile against stable Zig 0.16.0

[TanGentleman]

Problem

zig build currently fails on every released Zig toolchain:

• minimum_zig_version in build.zig.zon is pinned to 0.16.0-dev.205+4c0127566, a dev tarball that is no longer hosted on ziglang.org.

• docs/Walk.zig switches on Ast.Node.Tag.asm_legacy, which was removed before the 0.16.0 release. Building with stable 0.16.0 errors out:

  docs/Walk.zig:794:10: error: enum 'zig.Ast.Node.Tag' has no member named 'asm_legacy'
          .asm_legacy => {},
  

This is the same class of breakage as #5 / #6, just one Zig cycle later.

Fix

1. Bump minimum_zig_version to the released 0.16.0.
2. Drop the .asm_legacy => {}, arm. The arm was a no-op, and the surrounding switch is exhaustive — keeping it is a hard compile error on any Zig that no longer carries the enum value.

Verification

$ zig version
0.16.0
$ zig build
$ ls zig-out/main.wasm
zig-out/main.wasm

Built main.wasm exercised end-to-end via the downstream zigpeek CLI (search_std_lib, get_std_lib_item, builtin lookups) — no behavior change.

Notes

• I have been learning Zig, and had a really great time working on the zigpeek project built off the work in this repo! Cheers :D