Add inline header/header-lines/footer borders inside the list frame

Adds a new BorderShape, BorderInline, accepted as a value for
--header-border, --header-lines-border, and --footer-border. When the
surrounding --list-border has both top and bottom horizontals (rounded,
sharp, bold, double, horizontal), the corresponding section is rendered
inside the list frame separated from the list content by a horizontal
line whose endpoints join the list border as T-junctions. Without a
compatible list border, the shape falls back to BorderLine.

Supports:
  - All three layouts (default, reverse, reverse-list).
  - Any combination of the three inline sections, producing stacked
    separators.
  - --header-label and --footer-label rendered on their separator row
    (and redrawn on reqRedrawHeaderLabel / reqRedrawFooterLabel).
  - Section-specific border colors on the separator line, with the
    T-junction characters painted in the list-border color so the outer
    frame stays visually continuous.

Rejects the combinations that do not make sense:
  - --input-border=inline / --list-border=inline / --preview-border=inline
  - --header-first + (--header-border=inline | --header-lines-border=inline)
  - --header-border=inline with a non-inline --header-lines-border
    (inline has to propagate inward toward the list content).

test/test_layout.rb

@@ -1392,5 +1392,37 @@ class TestLayout < TestInteractive
                     input: "(printf 'Xaa\\nYbb\\nZcc\\n'; seq 5)",
                     clicks: clicks)
     end
+
+    # Inline header inside a rounded list border.
+    define_method(:"test_click_header_border_inline_#{slug}") do
+      opts = %(--layout=#{layout} --style full --header $'Aaa\\nBbb\\nCcc' )
+      verify_clicks(kind: :header, opts: opts, input: 'seq 5', clicks: HEADER_CLICKS)
+    end
+
+    # Inline header inside a horizontal list border (top+bottom only, no T-junctions).
+    define_method(:"test_click_header_border_inline_horizontal_list_#{slug}") do
+      opts = %(--layout=#{layout} --style full --header $'Aaa\\nBbb\\nCcc' )
+      verify_clicks(kind: :header, opts: opts, input: 'seq 5', clicks: HEADER_CLICKS)
+    end
+
+    # Inline header-lines inside a rounded list border.
+    define_method(:"test_click_header_lines_border_inline_#{slug}") do
+      clicks_hl = if layout == 'default'
+                    [%w[Xaa 3], %w[Ybb 2], %w[Zcc 1]]
+                  else
+                    [%w[Xaa 1], %w[Ybb 2], %w[Zcc 3]]
+                  end
+      opts = %(--layout=#{layout} --style full --header-lines 3 )
+      verify_clicks(kind: :header, opts: opts,
+                    input: "(printf 'Xaa\\nYbb\\nZcc\\n'; seq 5)",
+                    clicks: clicks_hl)
+    end
+
+    # Inline footer inside a rounded list border.
+    define_method(:"test_click_footer_border_inline_#{slug}") do
+      opts = %(--layout=#{layout} --style full --footer $'Foo\\nBar\\nBaz' )
+      verify_clicks(kind: :footer, opts: opts, input: 'seq 5',
+                    clicks: [%w[Foo 1], %w[Bar 2], %w[Baz 3]])
+    end
   end
 end