Ensure correct coordinate ordering in selection file write (#4078)

## Description

Fix an issue where `write_selection_file` would create empty files when
selecting multiple lines. This occurred because the selection
coordinates were not properly ordered when writing to file.

## Problem

When selecting multiple lines from bottom to top , the resulting file
would be empty. This happened because:

1. Selection coordinates were used directly without proper ordering

2. `start()` and `end()` don't guarantee top-to-bottom, left-to-right
ordering

## Solution

Use `topLeft()` and `bottomRight()` methods to ensure correct coordinate
ordering when writing selection to file.


## Testing

Tested the following scenarios:

- Selecting multiple lines from top to bottom ✅

- Selecting multiple lines from bottom to top ✅


## Known Behavior

When selecting a single word, the entire line containing that word is
written to the file. This is consistent with the current terminal
behavior as noted in the [original
discussion](https://github.com/ghostty-org/ghostty/discussions/3594).

Fixes ghostty-org/ghostty#3594

src/Surface.zig

@@ -4293,11 +4293,16 @@ fn writeScreenFile(
             tmp_dir.deinit();
             return;
         };
+
+        // Use topLeft and bottomRight to ensure correct coordinate ordering
+        const tl = sel.topLeft(&self.io.terminal.screen);
+        const br = sel.bottomRight(&self.io.terminal.screen);
+
         try self.io.terminal.screen.dumpString(
             buf_writer.writer(),
             .{
-                .tl = sel.start(),
-                .br = sel.end(),
+                .tl = tl,
+                .br = br,
                 .unwrap = true,
             },
         );

src/terminal/PageList.zig

@@ -3413,6 +3413,16 @@ pub const Pin = struct {
         direction: Direction,
         limit: ?Pin,
     ) PageIterator {
+        if (build_config.slow_runtime_safety) {
+            if (limit) |l| {
+                // Check the order according to the iteration direction.
+                switch (direction) {
+                    .right_down => assert(self.eql(l) or self.before(l)),
+                    .left_up => assert(self.eql(l) or l.before(self)),
+                }
+            }
+        }
+
         return .{
             .row = self,
             .limit = if (limit) |p| .{ .row = p } else .{ .none = {} },