Provider refactor: 2-method base class, ProcessProvider, ProcessPicker

Collapse Provider interface from 9 virtual methods to 2 (read + size),
move providers to src/providers/, add name()/kind()/getSymbol() virtuals.
Replace FileProvider with BufferProvider, add ProcessProvider (Win32)
with module-based symbol resolution, wire ProcessPicker dialog, and
integrate getSymbol into pointer display and command row.

- Fix isReadable overflow for large addresses
- Guard deferred showSourcePicker/showTypeAutocomplete against stale edits
- 7/7 tests pass including 3 new provider test suites

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/processpicker.h

@@ -0,0 +1,46 @@
+#ifndef PROCESSPICKER_H
+#define PROCESSPICKER_H
+
+#include <QDialog>
+#include <QIcon>
+#include <cstdint>
+
+namespace Ui {
+class ProcessPicker;
+}
+
+struct ProcessInfo {
+    uint32_t pid;
+    QString name;
+    QString path;
+    QIcon icon;
+};
+
+class ProcessPicker : public QDialog
+{
+    Q_OBJECT
+
+public:
+    explicit ProcessPicker(QWidget *parent = nullptr);
+    ~ProcessPicker();
+
+    uint32_t selectedProcessId() const;
+    QString selectedProcessName() const;
+
+private slots:
+    void refreshProcessList();
+    void onProcessSelected();
+    void filterProcesses(const QString& text);
+
+private:
+    void enumerateProcesses();
+    void populateTable(const QList<ProcessInfo>& processes);
+    void applyFilter();
+
+    Ui::ProcessPicker *ui;
+    uint32_t m_selectedPid = 0;
+    QString m_selectedName;
+    QList<ProcessInfo> m_allProcesses;
+};
+
+#endif // PROCESSPICKER_H