feat(simulation): persist SelectedBookmark across save/load (#863, #862)

Adds Serialize/Deserialize to SelectedBookmark and wires it into
SaveStateV1 so a loaded game remembers which bookmark and starting
location the player picked. Replaces the TODO at bookmark/mod.rs:95
(originally deferred to Sprint 37 alongside #614).

Also refactors BookmarkPlugin to accept an injected BookmarkRegistry
via BookmarkPlugin::new(registry) (#862). The Default constructor
still wires the canonical tycoon registry — injection is for tests
and future TOML loading. Flagged in PR #132 review as a follow-up.

Updates bookmark spec §4.4 to remove the v0.2-deferred scope note.

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

server/src/simulation/save_state.rs

@@ -51,6 +51,7 @@ use crate::npc::{
     PersonalityTraits, Relationships, Secret, SecretSeverity, SkillSet, TellSystem,
     ToleranceThreshold, Want, WantKind,
 };
+use crate::bookmark::SelectedBookmark;
 use crate::simulation::modification::Modification;
 use crate::simulation::movement::TilePosition;
 use crate::simulation::time::TickRate;
@@ -122,6 +123,11 @@ pub struct SaveStateV1 {
     /// `None` if no activation yet. Persisted alongside `activated_count`.
     #[serde(default)]
     pub last_activation_tick: Option<u64>,
+    /// Bookmark and starting location confirmed by the player at session start (#863, #614).
+    /// Both fields are `None` in saves created before #863 or before character creation
+    /// completes. Defaults to `SelectedBookmark::default()` for backward compatibility.
+    #[serde(default)]
+    pub selected_bookmark: SelectedBookmark,
 }
 
 /// Per-NPC state snapshot for `SaveStateV1`.
@@ -435,6 +441,7 @@ mod tests {
             contamination_active: false,
             activated_count: 0,
             last_activation_tick: None,
+            selected_bookmark: crate::bookmark::SelectedBookmark::default(),
         }
     }
 
@@ -854,6 +861,7 @@ mod tests {
             contamination_active: false,
             activated_count: 0,
             last_activation_tick: None,
+            selected_bookmark: crate::bookmark::SelectedBookmark::default(),
         };
 
         let bytes = save.to_bytes().expect("serialize");
@@ -878,6 +886,60 @@ mod tests {
         assert!(result.is_err(), "must panic without StableEntityId");
     }
 
+    // -----------------------------------------------------------------------
+    // SelectedBookmark round-trip (#863)
+    // -----------------------------------------------------------------------
+
+    #[test]
+    fn selected_bookmark_with_values_survives_roundtrip() {
+        // Spec (#863): SelectedBookmark persisted in SaveStateV1 must survive a
+        // full MessagePack serialize → deserialize cycle with field values intact.
+        let mut state = minimal_save_state();
+        state.selected_bookmark = crate::bookmark::SelectedBookmark {
+            bookmark_id: Some("tycoon".to_string()),
+            starting_location_id: Some("GJ 35".to_string()),
+        };
+
+        let bytes = state.to_bytes().expect("serialize");
+        let recovered = SaveStateV1::from_bytes(&bytes).expect("deserialize");
+
+        assert_eq!(
+            recovered.selected_bookmark.bookmark_id,
+            Some("tycoon".to_string()),
+            "bookmark_id must survive roundtrip"
+        );
+        assert_eq!(
+            recovered.selected_bookmark.starting_location_id,
+            Some("GJ 35".to_string()),
+            "starting_location_id must survive roundtrip"
+        );
+
+        // Idempotent re-serialize: bytes must be stable
+        let bytes2 = recovered.to_bytes().expect("re-serialize");
+        assert_eq!(bytes, bytes2, "SelectedBookmark roundtrip must be idempotent");
+    }
+
+    #[test]
+    fn selected_bookmark_default_survives_roundtrip() {
+        // Spec (#863): saves from before Sprint 37 (both fields None) must load
+        // cleanly via serde(default) on the SaveStateV1 field.
+        let state = minimal_save_state();
+        assert!(state.selected_bookmark.bookmark_id.is_none());
+        assert!(state.selected_bookmark.starting_location_id.is_none());
+
+        let bytes = state.to_bytes().expect("serialize");
+        let recovered = SaveStateV1::from_bytes(&bytes).expect("deserialize");
+
+        assert!(
+            recovered.selected_bookmark.bookmark_id.is_none(),
+            "default bookmark_id (None) must roundtrip"
+        );
+        assert!(
+            recovered.selected_bookmark.starting_location_id.is_none(),
+            "default starting_location_id (None) must roundtrip"
+        );
+    }
+
     // -----------------------------------------------------------------------
     // Modifications stub round-trip (#567, D-111/D-112)
     // -----------------------------------------------------------------------