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.

tests/test_prefix_stable.c

@@ -210,7 +210,7 @@ static void test_vision(const char *root) {
   snprintf(cfg.workspace_root, sizeof(cfg.workspace_root), "%s", root);
   snprintf(cfg.session_path, sizeof(cfg.session_path), "%s/blind.jsonl", dir);
   snprintf(cfg.api_base, sizeof(cfg.api_base), "http://replay/v1");
-  snprintf(cfg.model, sizeof(cfg.model), "deepseek-flash");
+  snprintf(cfg.model, sizeof(cfg.model), "deepseek-v4-pro");
   snprintf(cfg.provider, sizeof(cfg.provider), "deepseek");
   cfg.protocol = CST_PROTO_NATIVE_EOS;
   cfg.max_steps = 4;
@@ -220,7 +220,7 @@ static void test_vision(const char *root) {
   EXPECT(a != NULL, "blind agent");
   if (!a)
     return;
-  EXPECT(cst_agent_sees_images(a) == 0, "deepseek-flash is text only");
+  EXPECT(cst_agent_sees_images(a) == 0, "deepseek-v4-pro is text only");
   EXPECT(cst_agent_attach_bytes(a, "shot.png", (const char *)png, pn, err,
                                 sizeof(err)) == 0,
          "attach to a blind model");