Add --header-border=inline / --header-lines-border=inline / --footer-border=inline

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, thinblock, block, 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.
  - Section colors: the portion of the list frame adjacent to an inline
    section (left/right verticals on the section's content rows plus the
    outer top/bottom edge + corners when the section is at the edge)
    inherits the section's --color *-border and *-bg, giving each section
    a uniform color block. The separator itself carries the section's
    colors since it acts as the section's inner edge.
  - When --color header-border / --color footer-border is not set, the
    inline section inherits --color list-border so the default palette
    stays coherent.
  - thinblock / block styles pick the horizontal char (top vs bottom)
    based on which side of the list content the separator sits on, so
    the thin line visually hugs the list content.

Rejects 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).

man/man1/fzf.1

@@ -1100,7 +1100,13 @@ Print header before the prompt line. When both normal header and header lines
 .TP
 .BI "\-\-header\-border" [=STYLE]
 Draw border around the header section. \fBline\fR style draws a single
-separator line between the header window and the list section.
+separator line between the header window and the list section. \fBinline\fR
+style embeds the header inside the list border frame, joined to the list
+section by a horizontal separator with T-junctions; it requires a
+line-drawing \fB\-\-list\-border\fR (rounded / sharp / bold / double /
+horizontal) and falls back to \fBline\fR otherwise. Not compatible with
+\fB\-\-header\-first\fR, and when \fB\-\-header\-lines\fR is also set
+\fB\-\-header\-lines\-border\fR must also be \fBinline\fR.
 
 .TP
 .BI "\-\-header\-label" [=LABEL]
@@ -1116,6 +1122,9 @@ Display header from \fB--header\-lines\fR with a separate border. Pass
 \fBnone\fR to still separate the header lines but without a border. To combine
 two headers, use \fB\-\-no\-header\-lines\-border\fR. \fBline\fR style draws
 a single separator line between the header lines and the list section.
+\fBinline\fR style embeds the header lines inside the list border frame
+with a T-junction separator; it requires a line-drawing
+\fB\-\-list\-border\fR and is not compatible with \fB\-\-header\-first\fR.
 
 .SS FOOTER
 
@@ -1129,7 +1138,10 @@ are not affected by \fB\-\-with\-nth\fR. ANSI color codes are processed even whe
 .TP
 .BI "\-\-footer\-border" [=STYLE]
 Draw border around the footer section. \fBline\fR style draws a single
-separator line between the footer and the list section.
+separator line between the footer and the list section. \fBinline\fR style
+embeds the footer inside the list border frame with a T-junction separator;
+it requires a line-drawing \fB\-\-list\-border\fR and falls back to
+\fBline\fR otherwise.
 
 .TP
 .BI "\-\-footer\-label" [=LABEL]