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/null_provider.h

@@ -0,0 +1,14 @@
+#pragma once
+#include "provider.h"
+
+namespace rcx {
+
+class NullProvider : public Provider {
+public:
+    int  size() const override { return 0; }
+    bool read(uint64_t, void*, int) const override { return false; }
+    // name() returns "" via base default -- triggers <Select Source> in command row
+    // kind() returns "File" via base default
+};
+
+} // namespace rcx