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/editor.h

@@ -99,6 +99,9 @@ private:
     };
     InlineEditState m_editState;
 
+    // ── Tab cycling state ──
+    EditTarget m_lastTabTarget = EditTarget::Value;
+
     // ── Reentrancy guards ──
     bool m_clampingSelection = false;
     bool m_updatingComment = false;