feat(simulation): NPC mood state machine (#323, D-024, D-035)

8-state NPC mood FSM driven by stress, time of day, and interaction events.
Adds MoodState component, derive_mood() pure function, and update_mood() Bevy
system wired into NpcPlugin. Adds Focused as 9th Mood content tag (D-035
Sprint 8 amendment). Syncs CurrentMood for Layer 4 dialogue selection.

31 unit tests covering priority ordering, boundary conditions, and system
integration. All random values route through SimRng (D-010 determinism).
No floats. No HashMap.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/bin/line_preview.rs

@@ -637,5 +637,6 @@ fn mood_str(m: &Mood) -> &'static str {
         Mood::Conflicted => "conflicted",
         Mood::Concerned => "concerned",
         Mood::Relieved => "relieved",
+        Mood::Focused => "focused",
     }
 }