Add non-destructive disconnect status line and tick-driven prediction policy

Item 1: mosh-style disconnect status line. When UDP packets stop arriving,
paint a single reverse-video cyan "[dosh] last contact Ns ago — reconnecting…"
on the terminal's bottom row using save/restore cursor (ESC7/ESC8) so the app
cursor never moves and full-screen TUIs are not corrupted. Cleared the instant a
packet resumes and refreshed on the status timer tick. Configurable via
client.toml `disconnect_status` (default on) and `DOSH_DISCONNECT_STATUS` env
override. State machine and rendered escape sequences are pure and unit-tested.

Item 2: re-evaluate the prediction display policy on the select-loop status tick
via `Predictor::refresh_policy`, so a latency spike flips speculation on (or off
on recovery) promptly instead of waiting for the next keystroke. Repaints only
when the on-screen result would change, so idle ticks are no-ops.

Tests: 8 new client unit tests (status state machine + overlay bytes, predictor
spike/recovery/idle). fmt + full test suite green. No wire-format change.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.rs

@@ -114,6 +114,12 @@ pub struct ClientConfig {
     pub use_ssh_agent: bool,
     #[serde(default)]
     pub forward_agent: bool,
+    /// Show a non-destructive, mosh-style status line on the bottom screen row
+    /// when UDP packets stop arriving ("[dosh] last contact Ns ago …"). Drawn
+    /// with save/restore cursor so it never moves the app cursor or corrupts a
+    /// full-screen TUI, and cleared the instant packets resume. Default on.
+    #[serde(default = "default_true")]
+    pub disconnect_status: bool,
     #[serde(default = "default_send_env")]
     pub send_env: Vec<String>,
     #[serde(default)]
@@ -165,6 +171,7 @@ impl Default for ClientConfig {
             identity_files: default_identity_files(),
             use_ssh_agent: true,
             forward_agent: false,
+            disconnect_status: true,
             send_env: default_send_env(),
             set_env: HashMap::new(),
         }