FABRIC.md item 4.4x: split the REPL prompt into its own bottom strip

Scope expanded from pure CANVAS-rectangle arithmetic (as originally
scoped) to also splitting the REPL prompt/input line out of the
scrollback box into an independent single-line strip, per Captain Bob's
explicit fold-in after the gap was reported (§25.0 rule 3) rather than
silently expanded.

vt100.c: VT100_BOX_ORIGIN_X/Y are no longer hardcoded per-arch literals --
both are now derived from fb_width()/fb_height() at vt100_enable_ttf()
time. New vt100_strip_draw() renders the bottom strip (gray border lines,
bright-white text) directly via the existing ttf_draw_glyph_cell()
rasterizer, independent of the box's own grid/cursor state. Border lines
are drawn after the glyph loop so an oversized cell can only be clipped
by them, never erase them.

console.c/console.h: console_fb_strip_draw() thin wrapper, matching the
existing console_fb_enable_ttf()/console_fb_scroll_*() pattern.

repl.c: builds a plain-text "[VMName] ok> <input>" mirror in
g_strip_prompt/strip_refresh(), refreshed on every keystroke (including
backspace) from sk_readline() -- already wired for item 4.4v, since
keyboard-typed characters will flow through the same console_getc() path
once that lands. Also widened sk_repl_step()/sk_repl_run()'s local input
buffer from a second, smaller 256-byte buffer to INPUT_BUFFER_SIZE
(1025), per 4.4w's decision.

Verified: three-arch clean QEMU boot + logs, amd64 screendump showing
the box and strip as two visually distinct regions with no visible
glyph/border clipping.

Punch list §25 item 4.4x complete.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

include/starkernel/console.h

@@ -90,6 +90,14 @@ void console_fb_enable_ttf(void);
 void console_fb_scroll_back(uint32_t n);
 void console_fb_scroll_fwd(uint32_t n);
 
+/**
+ * FABRIC.md items 4.4u/4.4w/4.4x: thin wrapper over vt100_strip_draw() --
+ * see that function's doc comment for the full contract (the REPL
+ * prompt/input strip, independent of the scrollback box). No-op if the
+ * framebuffer console was never initialized.
+ */
+void console_fb_strip_draw(const char *text);
+
 /**
  * Write a single character to serial console
  */

include/starkernel/vt100.h

@@ -100,6 +100,18 @@ void vt100_putc(char c);
 /** Feed a null-terminated string. */
 void vt100_puts(const char *s);
 
+/**
+ * FABRIC.md items 4.4u/4.4w/4.4x: draw the REPL prompt/input strip -- a
+ * single-line region pinned to the bottom of the screen, independent of
+ * the scrollback box and its grid/cursor. @p text is the full line to
+ * show (prompt plus whatever has been typed so far), plain text with no
+ * ANSI/SGR codes -- the strip renders everything in one fixed color
+ * (bright white on black, 4.4u). Horizontally scrolls (tail-anchored)
+ * once @p text exceeds the strip's width. No-op before TTF mode is
+ * active (vt100_enable_ttf() must have run first).
+ */
+void vt100_strip_draw(const char *text);
+
 /* -----------------------------------------------------------------------
  * Queries
  * --------------------------------------------------------------------- */