docs(viewer): document npm registry bootstrap

[Aymericr]

Closes #454.

What changed

• document the complete npm package set and loadPlugin(builtinPlugin) ordering
• add the missing packaged README for @pascal-app/nodes
• correct the stale WallNode points example to current start/end fields
• update the root architecture summary for the registry package split
• warn once when mounts with an empty node registry

Verification

• bun check
• bun run --filter @pascal-app/viewer build
• bun run --filter @pascal-app/nodes build
• clean npm install of the published core/viewer/editor/nodes package set in a temporary project
• external bootstrap registered 44 built-in kinds and confirmed wall is present

The temporary verification project was moved to Trash after the check.

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Note

Low Risk
Mostly README changes; the only runtime change is a mount-time console warning with no behavior change when the plugin is loaded correctly.

Overview
Documents how published Pascal apps should install @pascal-app/core, viewer, editor, and nodes, and await loadPlugin(builtinPlugin) before mounting <Viewer>. Root and package READMEs now describe the four-package split (@pascal-app/nodes for built-in definitions/renderers/systems vs registry contracts in core), add a new packages/nodes/README.md, and refresh the viewer quick start with a ready-gate example and expanded peer deps.

@pascal-app/core docs drop the old “systems live in core” framing, point built-in geometry/tools at @pascal-app/nodes, and fix the WallNode sample to start/end instead of points.

<Viewer> logs a one-time console.warn when nodeRegistry is empty so hosts missing the plugin get an explicit bootstrap hint.

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