fix: prevent side-by-side panic with narrow width or large tab width

When rendering a side-by-side diff, `SourceDimensions::new` could compute
a per-column content width that was not greater than the tab width. That
violates the precondition of `split_and_apply`, which asserts
`max_len > tab_width` so it can always fit at least one expanded tab and
make progress when wrapping. As a result difftastic aborted with a panic
on ordinary input, for example `difft --width 14 a b` or, even at the
default terminal width, `difft --tab-width 100 a b`.

A very small `--width` could also drive the total display width down to
two columns or fewer, underflowing the `display_width - SPACER.len()`
subtraction (previously guarded by an assertion that itself aborted).

Enforce both invariants in `SourceDimensions::new`: keep room for the
column spacer, and ensure each content column is wider than the tab
width. Narrow terminals now degrade gracefully instead of crashing.

tests/cli.rs

@@ -137,6 +137,28 @@ fn text_changes_at_end_doesnt_crash() {
     cmd.assert().success();
 }
 
+#[test]
+fn narrow_width_doesnt_crash() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--width=14")
+        .arg("sample_files/text_1.txt")
+        .arg("sample_files/text_2.txt");
+
+    cmd.assert().success();
+}
+
+#[test]
+fn large_tab_width_doesnt_crash() {
+    let mut cmd = get_base_command();
+
+    cmd.arg("--tab-width=100")
+        .arg("sample_files/text_1.txt")
+        .arg("sample_files/text_2.txt");
+
+    cmd.assert().success();
+}
+
 #[test]
 fn makefile_text_as_atom() {
     let mut cmd = get_base_command();