style: fmt + clippy cleanup after Phase B

cargo fmt regroups, plus three Phase-B clippy fixes:
- F-P-004: #[allow(clippy::type_complexity)] on the invoke_ast_cache Mutex.
- F-P-008: hoist `futures::stream::{self, TryStreamExt}` to the top-of-
  file imports to satisfy clippy::items_after_statements.
- F-P-005: rename `_legacy_offset` to `legacy_offset` + #[allow(dead_code)]
  to satisfy clippy's "field is pub but `_`-prefixed" check.

No behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/manager-core/src/api.rs

@@ -381,8 +381,10 @@ pub struct LogsQuery {
     /// silently ignored to keep older dashboards from 400'ing.
     #[serde(default)]
     pub cursor: Option<String>,
+    /// Legacy field — accepted and ignored so older dashboards don't 400.
     #[serde(default, rename = "offset")]
-    pub _legacy_offset: Option<i64>,
+    #[allow(dead_code)]
+    pub legacy_offset: Option<i64>,
 }
 
 const fn default_limit() -> i64 {