fix: restore build:wasm npm script

[mcollina]

#5954 renamed the llhttp build script from build:wasm to build:llhttp. Node.js core's tools/dep_updaters/update-undici.sh runs npm run build:wasm, so the automated undici update for v8.12.0 fails:

+ node deps/npm/bin/npm-cli.js run build:wasm
npm error Missing script: "build:wasm"

Failing run: https://github.com/nodejs/node/actions/runs/38540716262/job/115774489408#step:5:82

This PR adds build:wasm back, running the same command as build:llhttp, so downstream tooling keeps working. Its stdout matches the old script, which matters because Node saves it to lib/llhttp/wasm_build_env.txt.

We will need a patch release (v8.12.1) to unblock the Node.js update.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LFcdjGCJYJoZW6iJt2DUh3

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.57%. Comparing base (6f98fcc) to head (3392c13).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6009   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files         113      113           
  Lines       43679    43679           
=======================================
  Hits        39999    39999           
  Misses       3680     3680           

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