Build builtin demo VFS images from shell base

[brandonpayton]

Summary

• add a shared build-time loader for the built shell.vfs.zst image
• make Node, nginx, nginx+PHP, WordPress SQLite, and LAMP VFS builders load the shell VFS as their base before overlaying demo-specific files
• update WordPress/LAMP package metadata and run.sh build ordering so derived VFS images depend on shell-vfs

Purpose

Give a richer shell environment to the rest of the core demos.

Verification

• git diff --check
• esbuild parse/bundle checks for modified VFS builder scripts and shell-vfs-build.ts

I did not rebuild full VFS images in this checkout because shell.vfs.zst and most service binaries were not present locally; rebuilding would require broad dependency fetch/build work.

[github-actions]

Phase B-1 matrix build status — pr-634-staging

ABI v15. 9 built, 2 failed, 11 total.

Package	Arch	Status	Sha
kandelo-sdk	wasm32	built	a1310e54
ncurses	wasm32	built	a52ba236
posix-utils-lite	wasm32	built	5d37bf48
bash	wasm32	built	b49c0ea4
nethack	wasm32	built	0cba9be6
nethack-browser-bundle	wasm32	built	311f4ed1
rootfs	wasm32	built	d9a430b5
shell	wasm32	built	45da4d2f
lamp	wasm32	failed	—
node-vfs	wasm32	built	5e8aa8ca
wordpress	wasm32	failed	—

_{Auto-generated; replaced on each push. Raw data in the publish-status workflow artifact.}