v0.7.5: arrow keys, image request fix, deepseek-flash vision, musl CI

TUI keys: register both CSI and SS3 forms of the cursor/Home/End keys and
the xterm modifier sequences when terminfo lacks them, so an arrow sent in
the "other" form no longer turns into Esc + a typed letter. Up/Down move by
visual (soft-wrapped) row, and the draft survives a trip through history.
scripts/tui_keys drives the TUI in a pty to check all of this.

Images: text inside a multimodal content array is now a {"type":"text"}
part; DeepSeek rejected the bare string with HTTP 422. deepseek-flash
(and its deepseek-chat / deepseek-reasoner aliases) read images, so the
registry marks them vision; deepseek-v4-pro stays text only.

CI: models.c lost _DEFAULT_SOURCE in 0.7.0 and gmtime_r broke the musl
build; tests sleep with poll() instead of usleep for the same reason.

src/llm_openai.c

@@ -162,7 +162,14 @@ static void parts_add_text(cJSON *parts, cJSON *txt) {
     cJSON_Delete(txt);
     return;
   }
-  cJSON_AddItemToArray(parts, txt);
+  if (!txt->valuestring || !txt->valuestring[0]) {
+    cJSON_Delete(txt);
+    return;
+  }
+  cJSON *t = cJSON_CreateObject();
+  cJSON_AddStringToObject(t, "type", "text");
+  cJSON_AddItemToObject(t, "text", txt);
+  cJSON_AddItemToArray(parts, t);
 }
 
 /* One stored image as an image_url part, or NULL with why filled in. */