[Oracle] Parse sets lazily to slash importer peak memory

- Add a raw JSON scanner that splits the document into per-set byte ranges
  without materializing the JSON tree
- Keep only the raw document bytes and parse one set at a time in startImport
- Take readSetsFromByteArray by value so the wizard's buffer is moved, not copied
- Clear the retained raw data in releaseSetData()/clear()
- Cover the scanner and lazy parsing with tests

Took 2 minutes

oracle/src/raw_json_scanner.h

@@ -0,0 +1,60 @@
+#ifndef RAW_JSON_SCANNER_H
+#define RAW_JSON_SCANNER_H
+
+#include <QByteArray>
+#include <QList>
+#include <QString>
+
+namespace RawJson
+{
+
+struct SetRange
+{
+    /** @brief Byte offset of the set's object within the scanned buffer. */
+    qsizetype start = -1;
+    /** @brief Byte length of the set's object, including the surrounding braces. */
+    qsizetype length = 0;
+    /** @brief Number of entries in the set's "cards" array. */
+    int cardCount = 0;
+    QString code;
+    QString name;
+    QString type;
+    QString releaseDate;
+};
+
+struct ScanError
+{
+    bool isError() const
+    {
+        return !message.isEmpty();
+    }
+    QString message;
+};
+
+/**
+ * @brief Scans a full MTGJSON document without materializing the JSON tree.
+ *
+ * Splits the top-level "data" object into per-set byte ranges and reads each
+ * set's metadata directly from the raw bytes. The oracle importer can then
+ * parse one set at a time during import, keeping peak memory far below a single
+ * QJsonDocument::fromJson() over the whole file.
+ *
+ * The whole document is structurally validated while scanning (strings,
+ * escapes, braces, and a trailing-content check), so malformed input is
+ * rejected just like QJsonDocument::fromJson would.
+ *
+ * Following QJsonDocument::fromJson's convention, the parsed ranges are
+ * returned by value and any failure is reported through the @p error out
+ * parameter.
+ *
+ * @param json The raw MTGJSON document bytes.
+ * @param error Out parameter. Set to an error ScanError when the document
+ *              cannot be parsed, otherwise left empty. Passing a null
+ *              pointer disables error reporting.
+ * @return The detected per-set ranges, or an empty list on failure.
+ */
+QList<SetRange> scanSetRanges(const QByteArray &json, ScanError *error = nullptr);
+
+} // namespace RawJson
+
+#endif // RAW_JSON_SCANNER_H