feat(merge): Rider-style 3-pane conflict editor view

Replace the Base|Ours|Theirs read-only columns + single-conflict result with a
whole-file 3-pane editor: Ours (read-only) | editable Result | Theirs (read-only),
reconstructed from the active file's segments so the panes line up on stable text.

- IBackgroundRenderer paints each conflict block (unresolved=blood, resolved=green)
  across all three panes.
- Result document edits are gated by an IReadOnlySectionProvider (stable text is
  read-only; only conflict regions, tracked via TextAnchors, are editable); edits
  flow back to the owning block.
- Between-pane gutters host inline accept controls (>/< ) positioned per conflict;
  click accepts ours/theirs into the result.
- Proportional synced vertical scroll across the panes; file switcher + change-nav
  arrows (F8 / Shift+F8); active-file 'M conflicts - K resolved' readout.
- Merge block tints + AmberBrush tokens; en/de keys for the new labels.

Seam unchanged. App builds; Ui.Tests 128, Localization.Tests 16.

src/ClaudeDo.Localization/locales/en.json

@@ -400,13 +400,14 @@
     "windowTitle": "Resolve merge conflicts",
     "modalTitle": "RESOLVE CONFLICTS",
     "loading": "Loading conflicts…",
-    "current": "Current (ours)",
-    "incoming": "Incoming (theirs)",
-    "mergedResult": "Merged result",
-    "acceptCurrent": "Accept Current",
-    "acceptIncoming": "Accept Incoming",
-    "acceptBoth": "Accept Both",
-    "editManually": "Edit manually",
+    "ours": "OURS · current (merge target)",
+    "result": "RESULT",
+    "theirs": "THEIRS · incoming (task)",
+    "binaryHint": "Binary files can't be merged here — abort and resolve them in your editor:",
+    "prevConflict": "Previous conflict (Shift+F8)",
+    "nextConflict": "Next conflict (F8)",
+    "acceptOurs": "Accept ours into result",
+    "acceptTheirs": "Accept theirs into result",
     "continue": "Resolve & continue",
     "abort": "Abort merge"
   },