feat(persistence): SQLite factory + versioned Migrator

src/VolcanicCore.Persistence/Migrations/IMigration.cs

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+using Microsoft.Data.Sqlite;
+
+namespace VolcanicCore.Persistence.Migrations;
+
+public interface IMigration
+{
+    int Version { get; }             // starts at 1, strictly monotonic
+    void Apply(SqliteConnection conn);
+}

src/VolcanicCore.Persistence/Migrator.cs

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+using Microsoft.Data.Sqlite;
+using VolcanicCore.Persistence.Migrations;
+
+namespace VolcanicCore.Persistence;
+
+public sealed class Migrator
+{
+    private readonly IReadOnlyList<IMigration> _migrations;
+
+    public Migrator(IEnumerable<IMigration> migrations)
+    {
+        _migrations = migrations.OrderBy(m => m.Version).ToArray();
+        for (var i = 1; i < _migrations.Count; i++)
+            if (_migrations[i].Version == _migrations[i - 1].Version)
+                throw new InvalidOperationException($"Duplicate migration version {_migrations[i].Version}");
+    }
+
+    public void Apply(SqliteConnection conn)
+    {
+        var current = ReadUserVersion(conn);
+        foreach (var m in _migrations)
+        {
+            if (m.Version <= current) continue;
+            using var tx = conn.BeginTransaction();
+            m.Apply(conn);
+            using (var cmd = conn.CreateCommand())
+            {
+                cmd.Transaction = tx;
+                cmd.CommandText = $"PRAGMA user_version = {m.Version};";
+                cmd.ExecuteNonQuery();
+            }
+            tx.Commit();
+        }
+    }
+
+    private static int ReadUserVersion(SqliteConnection conn)
+    {
+        using var cmd = conn.CreateCommand();
+        cmd.CommandText = "PRAGMA user_version;";
+        return Convert.ToInt32(cmd.ExecuteScalar());
+    }
+}

src/VolcanicCore.Persistence/SqliteConnectionFactory.cs

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+using Microsoft.Data.Sqlite;
+
+namespace VolcanicCore.Persistence;
+
+public sealed class SqliteConnectionFactory
+{
+    private readonly string _connectionString;
+
+    public SqliteConnectionFactory(string filePath)
+    {
+        _connectionString = new SqliteConnectionStringBuilder
+        {
+            DataSource = filePath,
+            Cache = SqliteCacheMode.Shared,
+            Pooling = true
+        }.ToString();
+    }
+
+    public SqliteConnection Open()
+    {
+        var conn = new SqliteConnection(_connectionString);
+        conn.Open();
+        using (var cmd = conn.CreateCommand())
+        {
+            cmd.CommandText = "PRAGMA journal_mode = WAL; PRAGMA foreign_keys = ON; PRAGMA busy_timeout = 5000;";
+            cmd.ExecuteNonQuery();
+        }
+        return conn;
+    }
+}

tests/VolcanicCore.Tests/MigratorTests.cs

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+using Microsoft.Data.Sqlite;
+using VolcanicCore.Persistence;
+using VolcanicCore.Persistence.Migrations;
+
+namespace VolcanicCore.Tests;
+
+public class MigratorTests
+{
+    private sealed class M1 : IMigration
+    {
+        public int Version => 1;
+        public void Apply(SqliteConnection conn)
+        {
+            using var cmd = conn.CreateCommand();
+            cmd.CommandText = "CREATE TABLE t (id INTEGER PRIMARY KEY);";
+            cmd.ExecuteNonQuery();
+        }
+    }
+
+    private sealed class M2 : IMigration
+    {
+        public int Version => 2;
+        public void Apply(SqliteConnection conn)
+        {
+            using var cmd = conn.CreateCommand();
+            cmd.CommandText = "ALTER TABLE t ADD COLUMN name TEXT;";
+            cmd.ExecuteNonQuery();
+        }
+    }
+
+    [Fact]
+    public void Migrator_applies_missing_migrations_in_order_and_sets_user_version()
+    {
+        using var conn = new SqliteConnection("Data Source=:memory:");
+        conn.Open();
+        var mig = new Migrator(new IMigration[] { new M2(), new M1() });
+
+        mig.Apply(conn);
+
+        Assert.Equal(2, ReadUserVersion(conn));
+        // Re-applying is a no-op.
+        mig.Apply(conn);
+        Assert.Equal(2, ReadUserVersion(conn));
+    }
+
+    private static int ReadUserVersion(SqliteConnection conn)
+    {
+        using var cmd = conn.CreateCommand();
+        cmd.CommandText = "PRAGMA user_version;";
+        return Convert.ToInt32(cmd.ExecuteScalar());
+    }
+}