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/providers/buffer_provider.h

@@ -0,0 +1,47 @@
+#pragma once
+#include "provider.h"
+#include <QFile>
+#include <QFileInfo>
+
+namespace rcx {
+
+class BufferProvider : public Provider {
+    QByteArray m_data;
+    QString    m_name;
+
+public:
+    explicit BufferProvider(QByteArray data, const QString& name = {})
+        : m_data(std::move(data))
+        , m_name(name) {}
+
+    static BufferProvider fromFile(const QString& path) {
+        QFile f(path);
+        if (f.open(QIODevice::ReadOnly))
+            return BufferProvider(f.readAll(), QFileInfo(path).fileName());
+        return BufferProvider({});
+    }
+
+    int size() const override { return m_data.size(); }
+
+    bool read(uint64_t addr, void* buf, int len) const override {
+        if (!isReadable(addr, len)) return false;
+        std::memcpy(buf, m_data.constData() + addr, len);
+        return true;
+    }
+
+    bool isWritable() const override { return true; }
+
+    bool write(uint64_t addr, const void* buf, int len) override {
+        if (!isReadable(addr, len)) return false;
+        std::memcpy(m_data.data() + addr, buf, len);
+        return true;
+    }
+
+    QString name() const override { return m_name; }
+    QString kind() const override { return QStringLiteral("File"); }
+
+    const QByteArray& data() const { return m_data; }
+    QByteArray& data() { return m_data; }
+};
+
+} // namespace rcx