Initial commit: Theriapolis baseline at port/godot branch point

Captures the pre-Godot-port state of the codebase. This is the rollback
anchor for the Godot port (M0 of theriapolis-rpg-implementation-plan-godot-port.md).
All Phase 0 through Phase 6.5 work is included; Phase 7 is in flight.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Theriapolis.Core/Persistence/SaveMigrations/V6ToV7Migration.cs

@@ -0,0 +1,30 @@
+namespace Theriapolis.Core.Persistence.SaveMigrations;
+
+/// <summary>
+/// Phase 6.5 M0 — additive migration from save schema v6 (Phase 6 ship) to
+/// v7 (Phase 6.5 levelling). Non-destructive: every v6 field carries over
+/// unchanged. The new <see cref="PlayerCharacterState.SubclassId"/>,
+/// <see cref="PlayerCharacterState.LearnedFeatureIds"/>, and
+/// <see cref="PlayerCharacterState.LevelUpHistory"/> default-initialise
+/// to empty values by the constructor, so this migration just bumps the
+/// header version.
+///
+/// Phase-6 saves had no level-up history (every character stayed at level
+/// 1, Xp = 0). On load they continue to be valid level-1 characters; the
+/// player can immediately start earning XP and levelling up under the new
+/// rules.
+/// </summary>
+public sealed class V6ToV7Migration : ISaveMigration
+{
+    public int FromVersion => 6;
+    public int ToVersion   => 7;
+
+    public void Apply(SaveHeader header, SaveBody body)
+    {
+        // No data translation needed. PlayerCharacterState's new fields
+        // (SubclassId, LearnedFeatureIds, LevelUpHistory) default-initialise
+        // to empty in their record, and SaveCodec.ReadCharacter handles
+        // missing-section bytes via the EOS-check pattern Phase 5 already
+        // established. Pure version bump.
+    }
+}