refactor(mtext): parse MTEXT via acadrust, drop OCS's own parser

OCS maintained a second MTEXT inline-code parser that duplicated
acadrust's `mtext_format::parse_mtext`. Replace it with an adapter
(`adapt_mtext_paragraphs`) that maps acadrust's structured MTextDocument
onto the existing layout types (`MTextLine`/`RunState`); the layout
engine (`layout_mtext`, `mtext_line_count`, and their callers) is
unchanged. Removed the hand-rolled `parse_mtext_paragraphs*` and its
helpers.

Validated during migration by a differential test (adapter vs the old
parser matched byte-for-byte over a 20-case corpus) and confirmed
rendering identically in the app; the differential test is replaced with
adapter-only unit tests. Relies on acadrust's new `MTextScalar` to tell
absolute `\H` from relative `\Hx` (bumped the acadrust pin).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.lock

@@ -71,7 +71,7 @@ checksum = "366ffbaa4442f4684d91e2cd7c5ea7c4ed8add41959a31447066e279e432b618"
 [[package]]
 name = "acadrust"
 version = "0.4.0"
-source = "git+https://github.com/HakanSeven12/acadrust?branch=main#4d519473ee1c03275be87d9c3bda4ddc53db45b0"
+source = "git+https://github.com/HakanSeven12/acadrust?branch=main#eae0ec3ca1777c1a89d78374fe56a1edc130e5ed"
 dependencies = [
  "ahash 0.8.12",
  "anyhow",
@@ -3160,7 +3160,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0b577e2d69827c4740cba2b52efaad1c4cc7c73042860b199710b3575c68438d"
 dependencies = [
  "bytecount",
- "memchr 1.0.2",
+ "memchr 2.8.2",
  "nom 8.0.0",
 ]