Use AUTOINCREMENT so generated ids are never reused (#6)

Plain INTEGER PRIMARY KEY reuses the rowid of a deleted highest row on the
next insert. Declaring the id column as INTEGER PRIMARY KEY AUTOINCREMENT
makes SQLite track the highest id ever assigned in sqlite_sequence and
never hand it out again, which is the expected behavior for stable record
identifiers.

- CreateTableQuery emits PRIMARY KEY AUTOINCREMENT for the id column
- Added a test asserting an id is not reused after the highest row is
  deleted
- README documents the no-reuse guarantee

Author: claude

README.md

@@ -182,7 +182,7 @@ people.push_back({L"Jane", L"Doe", 41, true, 7});
 db.Save(people);
 ```
 
-Use `SaveAutoIncrement` when you want the library to assign the next available `id` for the saved row. The generated `id` is written back into the passed-in object, so it must be a mutable (non-`const`) lvalue.
+Use `SaveAutoIncrement` when you want the library to assign the next available `id` for the saved row. The database generates the `id` (via an `AUTOINCREMENT` column, so values are never reused even after rows are deleted) and it is written back into the passed-in object, which must therefore be a mutable (non-`const`) lvalue.
 
 ```c++
 // Omit the id and let sqlite-reflection assign the next available value.

src/queries.cc

@@ -194,7 +194,9 @@ std::string CreateTableQuery::CustomizedColumnName(size_t index) const {
     const auto is_id = name.compare(std::string("id")) == 0;
     name += " " + record_.member_metadata[index].sqlite_column_name;
 
-    return is_id ? name + " PRIMARY KEY" : name;
+    // AUTOINCREMENT guarantees that ids are never reused, even after the row with the
+    // highest id is deleted
+    return is_id ? name + " PRIMARY KEY AUTOINCREMENT" : name;
 }
 
 DeleteQuery::DeleteQuery(sqlite3* db, const Reflection& record, const QueryPredicateBase* predicate)

tests/database_test.cc

@@ -120,6 +120,26 @@ TEST_F(DatabaseTest, AutoIdIncrementContinuesFromExistingMaxId) {
     EXPECT_EQ(11, p.id);
 }
 
+TEST_F(DatabaseTest, AutoIdIsNotReusedAfterDeletingHighestRow) {
+    const auto& db = Database::Instance();
+
+    Person first{L"ada", L"lovelace", 36};
+    db.SaveAutoIncrement(first);
+    EXPECT_EQ(1, first.id);
+
+    Person second{L"grace", L"hopper", 85};
+    db.SaveAutoIncrement(second);
+    EXPECT_EQ(2, second.id);
+
+    // Remove the row with the highest id
+    db.Delete(second);
+
+    // A subsequent auto-increment save must not reuse the freed id
+    Person third{L"john", L"doe", 28};
+    db.SaveAutoIncrement(third);
+    EXPECT_EQ(3, third.id);
+}
+
 TEST_F(DatabaseTest, MultipleInsertions) {
     const auto& db = Database::Instance();