Add mosh-grade predictive local echo

Replace the basic printable-only predictor with a clean-room speculative
local-echo engine modeled on the Mosh paper's prediction overlay design
(no GPL code copied or translated; original Rust written from the
algorithm).

Model:
- Per-line cell model with a local cursor; printable keystrokes echo
  immediately, dimmed (and underlined on a very slow link) via
  save/restore cursor so full-screen TUIs are never corrupted.
- Backspace and in-line Left/Right arrow prediction (CSI and SS3).
- Epoch grouping per keystroke burst; anything unmodelable (CR/LF,
  escapes, control bytes, wide chars, right margin, paste-sized bursts)
  ends the epoch safely rather than mispredicting.
- Frame-sequenced confirmation: an authoritative server frame supersedes
  and erases the overlay (dosh has no client-side emulator, so it cannot
  do Mosh's per-cell content match; frame arrival is the confirmation).
- Glitch detection: contradicting output (e.g. alternate-screen entry)
  discards the epoch.
- SRTT-based display policy with hysteresis and a latency-spike override;
  modes off / experimental(adaptive, default) / always via new
  client config `predict_mode` and `DOSH_PREDICT_MODE` env override.

Improvements over the prior predictor: in-line arrow motion, dim+flag
styling, exact-width erase, paste guard, and faster (frame-arrival)
confirmation. Prediction is display-only and never swallows input.

Adds unit tests: printable run, confirmation-clears, glitch-discards,
backspace, line-wrap budget, paste-skip, newline, inline arrows, and the
experimental SRTT threshold gate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.rs

@@ -84,6 +84,10 @@ pub struct ClientConfig {
     pub view_only: bool,
     #[serde(default)]
     pub predict: bool,
+    /// Prediction display policy: "off", "experimental" (adaptive, the default),
+    /// or "always". Controls when speculative local echo is shown.
+    #[serde(default = "default_predict_mode")]
+    pub predict_mode: String,
     pub cache_attach_tickets: bool,
     pub credential_cache: String,
     #[serde(default = "default_auth_preference")]
@@ -141,6 +145,7 @@ impl Default for ClientConfig {
             reconnect_timeout_secs: 5,
             view_only: false,
             predict: false,
+            predict_mode: default_predict_mode(),
             cache_attach_tickets: true,
             credential_cache: "~/.local/share/dosh/credentials".to_string(),
             auth_preference: default_auth_preference(),
@@ -183,6 +188,10 @@ fn default_native_auth_timeout_ms() -> u64 {
     700
 }
 
+fn default_predict_mode() -> String {
+    "experimental".to_string()
+}
+
 fn default_known_hosts() -> String {
     "~/.config/dosh/known_hosts".to_string()
 }