chore(core): tidy review nits — alphabetical lib.rs, import placement, docstring

End-of-phase cleanup of items flagged across review passes for tasks
1.1, 1.2, and 1.3:

1. crates/core/src/lib.rs: pub mod power moved to alphabetical
   position (between paths and process_watch).
2. crates/core/src/power.rs: use statements moved to top of file
   (above the PowerState enum) per Rust convention.
3. crates/core/src/power.rs: parse_power_state_from_dir docstring
   tightened to acknowledge that read_dir.flatten() silently skips
   per-entry errors. Theoretical hazard only on world-readable
   sysfs, but the previous wording overstated the guarantee.

No behavioural change. All 40 magnotia-core tests still green.

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

crates/core/src/lib.rs

@@ -3,9 +3,9 @@ pub mod error;
 pub mod hardware;
 pub mod model_registry;
 pub mod paths;
+pub mod power;
 pub mod process_watch;
 pub mod recommendation;
-pub mod power;
 pub mod tuning;
 pub mod types;