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/controller.h

@@ -86,6 +86,7 @@ public:
 
 signals:
     void nodeSelected(int nodeIdx);
+    void selectionChanged(int count);
 
 private:
     RcxDocument*       m_doc;
@@ -97,6 +98,7 @@ private:
     void connectEditor(RcxEditor* editor);
     void handleMarginClick(RcxEditor* editor, int margin, int line, Qt::KeyboardModifiers mods);
     void updateCommandRow();
+    void attachToProcess(uint32_t pid, const QString& processName);
 };
 
 } // namespace rcx