host: preserve journals, full traps and embedded agent output

Write new journal entries under logs/core and merge read-only legacy
history without moving SQLite/WAL files. Atomically spool failed batches,
retaining unspoolable entries for event-driven retries. Journal full trap
diagnostics before UTF-8-safe status display. Port session overflow archives
and followable line/byte excerpts to the still-shipped embedded agent.
Document retention/recovery and cover the contracts with regressions.
Include one rustfmt-only normalization in the existing SDK WASI test.

Card: SAU-143

sdk/tests/wasi.rs

@@ -34,10 +34,8 @@ impl Process {
             let mut s = String::new();
             if self.output.read_line(&mut s).unwrap() == 0 {
                 let mut stderr = String::new();
-                let _ = std::io::Read::read_to_string(
-                    self.child.stderr.as_mut().unwrap(),
-                    &mut stderr,
-                );
+                let _ =
+                    std::io::Read::read_to_string(self.child.stderr.as_mut().unwrap(), &mut stderr);
                 panic!("unexpected EOF from wasmtime. Its stderr:\n{stderr}");
             }
             if let Ok(v) = sausage_plugin::serde_json::from_str::<Value>(&s)