Sprint 277: LSP Advanced Orchestration (steps 1913-1917)

- LSPWorkspaceIndex: track open docs, URI→{languageId,content,version}
- LSPDiagnosticsAggregator: merge diagnostics from multiple LSP sources per URI
- LSPCompletionMerger: merge+deduplicate completion items from multiple sources
- LSPOrchestrator: top-level coordinator composing all three + proxy + cross-lang resolver
- Sprint277IntegrationSummary: poly-api (Go+TypeScript) end-to-end integration

26/26 tests passing (5+5+5+5+6). All new headers ≤600 lines.
LoRA recorded: sprint277-2026-03-01 (1 call, 569 tokens).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

editor/src/LSPCompletionMerger.h

@@ -0,0 +1,68 @@
+#pragma once
+#include <algorithm>
+#include <map>
+#include <string>
+#include <vector>
+
+namespace whetstone {
+
+struct CompletionItem {
+    std::string label;
+    int         kind   = 1;    // 1=Text 2=Method 3=Function 6=Variable etc.
+    std::string detail;
+    std::string source;        // language server that produced this item
+};
+
+class LSPCompletionMerger {
+public:
+    // Add items from a named source. Replaces any previous items from that source.
+    void addItems(const std::string& source,
+                  const std::vector<CompletionItem>& items) {
+        bySource_[source] = items;
+    }
+
+    // Return all items merged from all sources, deduplicated by label.
+    // When two sources offer the same label, the first encountered wins (sorted source order).
+    std::vector<CompletionItem> getItems() const {
+        std::vector<CompletionItem> result;
+        std::map<std::string, bool> seen;
+        for (const auto& [src, items] : bySource_) {
+            for (const auto& item : items) {
+                if (!seen.count(item.label)) {
+                    seen[item.label] = true;
+                    result.push_back(item);
+                }
+            }
+        }
+        std::sort(result.begin(), result.end(),
+                  [](const CompletionItem& a, const CompletionItem& b){
+                      return a.label < b.label;
+                  });
+        return result;
+    }
+
+    // Remove all items from one source.
+    void clearSource(const std::string& source) {
+        bySource_.erase(source);
+    }
+
+    // Remove all items.
+    void clearAll() {
+        bySource_.clear();
+    }
+
+    // Count unique labels across all sources.
+    int uniqueCount() const {
+        return static_cast<int>(getItems().size());
+    }
+
+    int sourceCount() const {
+        return static_cast<int>(bySource_.size());
+    }
+
+private:
+    // source → items
+    std::map<std::string, std::vector<CompletionItem>> bySource_;
+};
+
+} // namespace whetstone