Move LL-viz onto tensor-viz npm packages

[latentCall145]

Summary

• remove the tensor-viz git submodule
• make LL-viz a root Vite app that depends on @tensor-viz/viewer-core and @tensor-viz/viewer-demo
• move the linear-layout extension, presets, widgets, tests, and sync script into this repo
• update CI and Pages to build the root app; CI temporarily packs the tensor-viz extraction branch until the new npm packages are published

Verification

• installed local tarballs from tensor-viz PR Move LL-viz onto tensor-viz npm packages #1 with npm install --no-save ...
• npm run typecheck
• npm run test
• npm run build
• npm run sync:linear-layout-examples

Follow-up before final release

• merge tensor-viz PR Move LL-viz onto tensor-viz npm packages #1
• publish @tensor-viz/viewer-core and @tensor-viz/viewer-demo
• replace temporary CI pack/install steps with plain npm install