fix: resolve plant elevation through the host floor resolver

[Aymericr]

Problem

Trees, flowers and grass read node.position[1] raw when writing their transforms, so every plant sat at y = 0 — floating underneath a raised deck, and buried inside a sculpted hillside.

Why the host didn't handle it

Stored plant positions are flat by contract ([x, 0, z]) — the surface a plant stands on is resolved at render time, and the lift is presentation that is never committed.

For an ordinary per-node kind the host does this for free. But these kinds use a collective instanced renderer, and the host's FloorElevationSystem writes only to a node's registered object — which for an instanced kind is the invisible selection proxy, not the instances that are actually drawn. So an instanced kind has to resolve the lift itself, everywhere it writes a transform.

Change

New elevation.ts holds the seam, resolving through the host's getFloorStackedPosition so plants inherit slab election and sculpted terrain without knowing either exists:

• plantElevation — committed nodes. Used by the instance matrices in instanced.tsx, and by the proxy's box collider. The collider is a positioned sibling of the registered group (so the outline pass traces the plant's true silhouette instead of a box), which puts it outside the host's reach — without this, the hit volume stayed at the storey plane while the plant it stands for rode a deck or a hillside.
• draftElevation — the placement ghost. A draft is unparented, so the level it will land on has to be named explicitly (the resolver reads parentId first and only falls back to levelId). A ghost that skipped this floated at the storey plane over a deck and sank into every slope, so the plant appeared to jump on click.

Both live in a module free of any Three.js or React import, so the seam is testable without a canvas.

Depends on

Host support landed in pascalorg/editor#568 — the generic levelBaseElevationAt / ctx.levelBaseAt seam, where asking for the ground is what enrolls a kind in following it. This PR is that seam's first plugin consumer, so it doubles as the reference for how a third-party plugin inherits terrain.

Testing

elevation.test.ts (new) — 11 pass, 0 fail against the merged host core. Covers all three kinds resting on bare ground, riding a deck slab, and standing beyond the deck's footprint (the lift is per-position, not global), plus both ghost cases including an unresolvable level, which must stay flat rather than throw.

Worth noting: every assertion in the suite reads 0 under the old code. In a flat test scene the broken and correct implementations agree exactly — which is why this survived so long, and why the fixture builds an explicit deck and lets the spatial grid settle before asserting.

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