documents: edits read the caret off a CRLF split

The editor splits lines on LF only, so End can leave the caret
between CR and LF. The edit runner and table insert now settle the
selection before the CR; the line helpers assert they never see a
split instead of guessing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/document/apart.rs

@@ -6,7 +6,7 @@ use gpui::{AppContext as _, Context};
 
 use super::{
     DocumentEvent, LARGEST_READ, MarkdownDocument,
-    block::{self, BlockCommand},
+    block::{self, BlockCommand, settled},
     format::{Change, EditRefusal, InlineFormat, toggle},
     tables::{self, TableEdit},
 };
@@ -109,7 +109,8 @@ impl MarkdownDocument {
     fn snapshot(&self, cx: &Context<Self>) -> Snapshot {
         let text = self.editor.read(cx).text();
         let Some(open) = &self.open else {
-            return (text.to_string(), 0, self.editor.read(cx).primary().range());
+            let picked = self.editor.read(cx).primary().range();
+            return settle((text.to_string(), 0, picked));
         };
         let input = open.input.read(cx);
         let (before, after) = match open.append {
@@ -122,7 +123,7 @@ impl MarkdownDocument {
         let picked = input.selection();
         let offset = before.len();
         let full = format!("{before}{}{after}", input.text());
-        (full, offset, offset + picked.start..offset + picked.end)
+        settle((full, offset, offset + picked.start..offset + picked.end))
     }
 
     /// Writes `change`: into the open block while it stays inside.
@@ -179,3 +180,9 @@ impl MarkdownDocument {
         Ok(())
     }
 }
+
+/// The editor splits lines on LF only; reads start off a CRLF.
+fn settle((full, offset, picked): Snapshot) -> Snapshot {
+    let picked = settled(&full, picked.start)..settled(&full, picked.end);
+    (full, offset, picked)
+}