feat(mtext): editable preview with caret, blink, and new-line support

Type directly into the rendered MText preview: click to position the
caret, insert/delete, Enter for a new line, Space for a literal space,
and arrow keys to move. The caret blinks when idle.

Keep the trailing empty paragraph in the editor so the caret appears on a
fresh line immediately after Enter, before any typing. parse_mtext_paragraphs
split into a _ex variant; layout_mtext skips the blank-edge trim when
emitting glyph boxes.

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

Cargo.toml

@@ -5,7 +5,7 @@ edition = "2021"
 build = "build.rs"
 
 [dependencies]
-iced = { version = "0.14", features = ["debug", "image", "svg", "advanced", "canvas"] }
+iced = { version = "0.14", features = ["debug", "image", "svg", "advanced", "canvas", "smol"] }
 bytemuck = { version = "1.25", features = ["derive"] }
 glam = { version = "0.33", features = ["bytemuck"] }
 truck-modeling = "0.6"