feat!: adopt plugin API v2

[thecuriographer]

Adopts the three-manifest plugin contract (apiVersion: 2) that Pascal is
introducing, replacing the single NodeDefinition per kind.

What changes

Each of the three kinds now exports its parts separately:

Manifest	Owns
treesCoreManifest	schema, schemaVersion, defaults, semantic capabilities, relations
treesViewerManifest	geometry, renderers, instanced systems, bake policy
treesEditorManifest	parametrics, placement tools, plan symbols, palette entries

treesPlugin is gone; a host loads the three manifests with
loadCoreManifest, loadViewerManifest and loadEditorManifest. Core
registers the kind; the presentation manifests attach to it by kind and are
rejected if no base definition exists.

Why

NodeDefinition transitively named React and Three.js types, which is what
forced @pascal-app/core to re-export renderer types it does not own. With
those types gone from the contract, a plugin imports each concept from its
actual owner — sceneRegistry and useRegistry from @pascal-app/viewer,
GridEvent and ParametricDescriptor from @pascal-app/editor. That is not a
new dependency: this package already peer-depends on all three.

The split here is authored rather than derived. index.test.ts asserts it in
both directions — no semantic field (schema, defaults, capabilities, …)
appears in a presentation manifest, and no presentation field (renderer,
system, parametrics, floorplan, …) appears in a base definition — so a
field added to the wrong half fails the suite rather than silently working.

Compatibility

Breaking: needs a host on plugin API v2. Peer ranges move to >=1.0.0-beta.4.
Nothing in the authored scene format changes — schemas, schemaVersion and
node types are untouched, so existing scenes containing trees:tree,
trees:flower and trees:grass load unchanged.

Verification

bun test (17 tests) and tsc --noEmit both pass against Pascal's
plugin-api-v2 branch, with no type assertions anywhere in the manifests —
that last part was the acceptance bar for the contract itself, and it caught a
real flaw in the first draft of v2 (the manifest element types fixed the schema
generic, forcing a cast per kind; Pascal now erases it).