Rename Ratatoskr integration to Yggdrasil

[pyrex41]

Summary

• rename bin/ratatoskr-build.lua to bin/yggdrasil-build.lua
• update the builder and OpenResty browser pipeline to consume yggdrasil.manifest.txt and invoke yggdrasil
• update generated browser documentation, examples, links, and environment variables to Yggdrasil

Why

The Shen tree-shaker has returned to its original Yggdrasil name. The Lua stage-2 builder and example pipeline still invoked the former Ratatoskr CLI and manifest.

Impact

The stage-2 builder and browser example now use the Yggdrasil command, paths, manifest, and $YGGDRASIL override.

Checks

• parsed bin/yggdrasil-build.lua with LuaJIT
• shell syntax check for examples/openresty/scripts/build-client.sh
• Node syntax check for examples/openresty/scripts/build-client.mjs
• make test LUA=luajit — 896 passed, 0 failed
• case-insensitive source audit — no remaining ratatoskr matches
• git diff --cached --check