feat(viewer): per-level base elevation parameter

[mvanhorn]

What does this PR do?

Adds a per-level baseElevation parameter so each level can offset its computed stack position upward by a user-specified amount. The site panel surfaces a Base elevation MetricControl on each level (m, 2dp, 0.05 step) and the viewer's level system threads the offset through getLevelTargetY / getNextLevelCumulativeY helpers.

How to test

1. Open the site panel.
2. Expand any level.
3. Set the Base elevation field to a non-zero meter value.
4. Confirm the level (and all levels above) move up by that amount in 3D.
5. Switch to exploded mode; confirm EXPLODED_GAP still adds on top of the cumulative offset.
6. Run the test suite: bun test packages/core/test/level.test.ts packages/viewer/test/level-utils.test.ts.

Notes

check-types flags a pre-existing TypeScript error on the base branch unrelated to this change (packages/editor/src/store/use-editor.tsx:249 — childId implicit any). The error is present on origin/main before this branch is applied, so the failure isn't introduced by this PR. Happy to fix it in a sibling PR if useful.

Checklist

• Ran bun dev and exercised the change locally
• Ran bun check (biome clean)
• bun check-types fails on pre-existing error (see Notes)
• Tests added for the schema + utility paths
• Targets main

Closes #209