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.lock

@@ -328,6 +328,17 @@ dependencies = [
  "slab",
 ]
 
+[[package]]
+name = "async-fs"
+version = "2.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8034a681df4aed8b8edbd7fbe472401ecf009251c8b40556b304567052e294c5"
+dependencies = [
+ "async-lock",
+ "blocking",
+ "futures-lite",
+]
+
 [[package]]
 name = "async-io"
 version = "2.6.0"
@@ -357,6 +368,17 @@ dependencies = [
  "pin-project-lite",
 ]
 
+[[package]]
+name = "async-net"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b948000fad4873c1c9339d60f2623323a0cfd3816e5181033c6a5cb68b2accf7"
+dependencies = [
+ "async-io",
+ "blocking",
+ "futures-lite",
+]
+
 [[package]]
 name = "async-process"
 version = "2.5.0"
@@ -2092,6 +2114,7 @@ dependencies = [
  "iced_core",
  "log",
  "rustc-hash 2.1.2",
+ "smol",
  "wasm-bindgen-futures",
  "wasmtimer",
 ]
@@ -4873,6 +4896,23 @@ dependencies = [
  "wayland-backend",
 ]
 
+[[package]]
+name = "smol"
+version = "2.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a33bd3e260892199c3ccfc487c88b2da2265080acb316cd920da72fdfd7c599f"
+dependencies = [
+ "async-channel",
+ "async-executor",
+ "async-fs",
+ "async-io",
+ "async-lock",
+ "async-net",
+ "async-process",
+ "blocking",
+ "futures-lite",
+]
+
 [[package]]
 name = "smol_str"
 version = "0.2.2"