editor: drop the stale 6 m caps and surface the ceiling's level bound

[wass08]

What does this PR do?

Follow-up to #642, which raised the room-envelope height caps from 6 m to 20 m but left two sliders behind, so a tall storey still wasn't usable in practice.

• ceiling/panel.tsx — the non-level-parent fallback stayed at 6 while definition.ts uses Number.POSITIVE_INFINITY for the same case, so the panel and the 3D drag handle disagreed on the same bound. They now agree.
• slab/panel.tsx — the elevation sliders (Surface/Floor and Base/Rim) stayed at max={6}, so a slab could not be raised past 6 m inside a 20 m level. clampSlabElevation in the write path is the real bound; the slider was only a UI ceiling.

It also fixes a usability trap found while verifying the above. A ceiling's cap is the storey plane (getCeilingClampBound), which on a stock 2.5 m level is 2.49 m — so the "Standard (2.5m)" and "High (3.0m)" presets clamped silently and read as dead buttons, with nothing pointing at the level height as the actual gate. Presets taller than the level are now disabled with the available height in their tooltip, and the Height section names the bound.

The clamp itself is unchanged: a ceiling still never pokes through its own storey.

How to test

1. bun dev, draw a room with walls and a ceiling.
2. Select the ceiling → Height → switch to Custom height. On a default 2.5 m level, confirm the Standard (2.5m) and High (3.0m) presets are disabled, and hovering one explains that it's taller than the level.
3. Confirm the section now reads "Limited by the level to 2.49 m — raise the level height for a taller ceiling."
4. Raise the level height (level badge → Level height) to e.g. 6 m. The presets re-enable and the ceiling slider now goes up to 5.99 m.
5. Select a slab → Elevation. Confirm the Surface/Floor and Base/Rim sliders now travel to 20 m instead of stopping at 6 m, and that clampSlabElevation still holds them under the storey.

I verified the clamp empirically rather than by reading: getCeilingClampBound returns 19.99 on a 20 m level in every configuration tested — single level, level above, and level above with a covering floor slab.

Screenshots / screen recording

N/A — will add a short clip if the reviewer wants one; the visible change is two disabled preset buttons and one line of helper text in the ceiling panel.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch

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Note

Low Risk
Editor-only panel bounds and preset UX; geometry clamp logic is unchanged.

Overview
Removes leftover 6 m UI limits on ceiling and slab inspectors so tall storeys (up to 20 m) are editable in the panel, matching the write-path clamps and definition.ts.

In ceiling/panel.tsx, the non-level-parent maxHeight fallback changes from 6 to Number.POSITIVE_INFINITY (same as the 3D/registry bound). Height presets that exceed the storey clamp are disabled with a tooltip, and when the cap is finite the panel shows “Limited by the level to …” so silent clamping no longer looks like broken buttons.

In slab/panel.tsx, Surface/Floor and Base/Rim sliders max rise from 6 to 20; clampSlabElevation remains the real limit on save.

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