OmaPhoto 1.0.0: the Linux port of Compositor

A layered image editor for Linux and Omarchy, ported from Compositor
(github.com/robbietilton/Compositor), whose Swift source is its
specification. It reads and writes Compositor's .comp projects.

Packages for Ubuntu, Fedora and Arch come from the release workflow;
scripts/install/ holds one install script per distribution family,
and flake.nix serves NixOS.

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

src/Document/DocumentHistory.h

@@ -0,0 +1,55 @@
+#pragma once
+#include "Document/EditorSession+Model.h"
+#include <QString>
+#include <QUuid>
+#include <optional>
+#include <vector>
+
+// Snapshots share image data; layer edits copy no pixels.
+class DocumentHistory {
+public:
+    struct Snapshot {
+        std::optional<CanvasDocument> document;
+        std::optional<QUuid> activeLayerID;
+        QUuid revision;
+    };
+
+    const int entryLimit;
+    const qint64 retainedByteLimit;
+
+    explicit DocumentHistory(int entryLimit = 100, qint64 retainedByteLimit = 256LL * 1024 * 1024);
+
+    bool canUndo() const { return m_depth == 0 && !m_past.empty(); }
+    bool canRedo() const { return m_depth == 0 && !m_future.empty(); }
+    QString undoName() const { return m_past.empty() ? QString() : m_past.back().name; }
+    QString redoName() const { return m_future.empty() ? QString() : m_future.back().name; }
+    bool isModified() const { return m_revision != m_savedRevision; }
+    int undoCount() const { return int(m_past.size()); }
+    void markSaved() { m_savedRevision = m_revision; }
+    void reset();
+    // Takes the steps a staged copy of this history recorded.
+    void adopt(DocumentHistory &&staged) noexcept;
+
+    void begin(const QString &name, const std::optional<CanvasDocument> &document, std::optional<QUuid> selection);
+    void end(const std::optional<CanvasDocument> &document, std::optional<QUuid> selection);
+    std::optional<Snapshot> undo();
+    std::optional<Snapshot> redo();
+    qint64 retainedBytes(const std::optional<CanvasDocument> &current) const;
+
+private:
+    struct Entry {
+        QString name;
+        Snapshot before;
+        Snapshot after;
+    };
+
+    void trim(const std::optional<CanvasDocument> &current);
+
+    std::vector<Entry> m_past;
+    std::vector<Entry> m_future;
+    QUuid m_revision = QUuid::createUuid();
+    QUuid m_savedRevision = m_revision;
+    std::optional<Snapshot> m_pending;
+    QString m_pendingName = QStringLiteral("Edit");
+    int m_depth = 0;
+};