fix(mtext): correct imported-text size in the editor (#394)

The editor preview mis-sized imported MTEXT badly (tiny, huge, or
collapsed). Two root causes:

- The entity's base height often isn't the visible text height: relative
  `\H…x;` codes compound, so an entity of height 1 whose body is `\H0.2x`
  renders at 0.2. Normalize the preview scale by the DOMINANT rendered run
  height (the mode of glyph-box heights, skipping zero-width caret slots),
  not `height_value()`, so the body lands at a fixed readable size and
  headings keep their proportions. EM_PX = 15.
- The bug that made it "completely wrong" was in the serializer, fixed in
  acadrust 389b31a (\H heights decayed on the parse→serialize→parse the
  editor does via folded_value); bumped here.

Also render multi-column MTEXT with its columns side by side (as the
drawing does) and let the preview scroll horizontally to reach them.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

Cargo.lock

@@ -73,7 +73,7 @@ checksum = "366ffbaa4442f4684d91e2cd7c5ea7c4ed8add41959a31447066e279e432b618"
 [[package]]
 name = "acadrust"
 version = "0.4.0"
-source = "git+https://github.com/OpenAEC-Foundation/acadifc?branch=main#9d2e67fb7bfc054281641228e3543e8ac26d7f08"
+source = "git+https://github.com/OpenAEC-Foundation/acadifc?branch=main#389b31ac5865e72f403d16d81eec958228065af0"
 dependencies = [
  "ahash 0.8.12",
  "anyhow",
@@ -3179,7 +3179,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0b577e2d69827c4740cba2b52efaad1c4cc7c73042860b199710b3575c68438d"
 dependencies = [
  "bytecount",
- "memchr 1.0.2",
+ "memchr 2.8.3",
  "nom 8.0.0",
 ]