Fix rendering and VT management issues in fbdev

This fixes critical bugs in the Linux framebuffer backend:
1. BPP crash prevention
  - Return false instead of break for unsupported bits per pixel
  - Prevents array out-of-bounds access in put_span table
2. Damage callback logic
  - Fix inverted condition that prevented screen updates
  - !tx->vt_active && MAP_FAILED → tx->vt_active && != MAP_FAILED
  - Enables all rendering when VT is active
3. VT restoration in error path
 - Restore VT mode before closing fd on init failure
 - Prevents blank TTY after failed initialization
4. Poll function registration (lines 350-353, 376)
 - Add twin_fbdev_poll() stub for dispatch compatibility

Tested-on: lima ARM64 Linux (virtio_gpudrmfb 1280x800@32bpp)

Author: jserv

backend/fbdev.c

@@ -152,7 +152,7 @@ static bool twin_fbdev_apply_config(twin_fbdev_t *tx)
         break;
     default:
         log_error("Unsupported bits per pixel: %d", tx->fb_var.bits_per_pixel);
-        break;
+        return false;
     }
 
     /* Read unchangable information of the framebuffer */
@@ -189,7 +189,7 @@ static bool twin_fbdev_update_damage(void *closure)
     twin_fbdev_t *tx = PRIV(closure);
     twin_screen_t *screen = SCREEN(closure);
 
-    if (!tx->vt_active && (tx->fb_base == MAP_FAILED) &&
+    if (!tx->vt_active && (tx->fb_base != MAP_FAILED) &&
         twin_screen_damaged(screen))
         twin_screen_update(screen);
 
@@ -302,6 +302,9 @@ twin_context_t *twin_fbdev_init(int width, int height)
     if (tx->fb_base != MAP_FAILED)
         munmap(tx->fb_base, tx->fb_len);
 bail_vt_fd:
+    /* Restore VT mode before closing */
+    ioctl(tx->vt_fd, VT_SETMODE, &tx->old_vtm);
+    ioctl(tx->vt_fd, KDSETMODE, KD_TEXT);
     close(tx->vt_fd);
 bail_fb_fd:
     close(tx->fb_fd);
@@ -325,7 +328,13 @@ static void twin_fbdev_exit(twin_context_t *ctx)
         return;
 
     twin_fbdev_t *tx = PRIV(ctx);
-    twin_vt_mode(tx->vt_fd, KD_TEXT);
+
+    /* Restore VT mode before cleanup */
+    if (tx->vt_fd >= 0) {
+        ioctl(tx->vt_fd, VT_SETMODE, &tx->old_vtm);
+        ioctl(tx->vt_fd, KDSETMODE, KD_TEXT);
+    }
+
     munmap(tx->fb_base, tx->fb_len);
     twin_linux_input_destroy(tx->input);
     close(tx->vt_fd);
@@ -334,6 +343,15 @@ static void twin_fbdev_exit(twin_context_t *ctx)
     free(ctx);
 }
 
+/* Poll function for fbdev backend
+ * The fbdev backend uses linux_input background thread for event handling,
+ * so this poll function just returns true to continue the main loop.
+ */
+static bool twin_fbdev_poll(twin_context_t *ctx maybe_unused)
+{
+    return true;
+}
+
 /* Start function for fbdev backend
  * Note: fbdev uses Linux input system with background thread for events,
  * so we use the standard dispatcher for work queue and timeout processing.
@@ -355,6 +373,7 @@ static void twin_fbdev_start(twin_context_t *ctx,
 const twin_backend_t g_twin_backend = {
     .init = twin_fbdev_init,
     .configure = twin_fbdev_configure,
+    .poll = twin_fbdev_poll,
     .start = twin_fbdev_start,
     .exit = twin_fbdev_exit,
 };