Sprint 278: LSP Proxy Hardening (steps 1918-1922)

- LSPMessageValidator: JSON-RPC 2.0 shape validation (method, id type, params type)
- LSPCapabilityNegotiator: per-source capability tracking; shouldRoute gates on declared caps
- LSPRequestThrottler: per-URI/method debounce; blocks rapid didChange floods
- LSPErrorRecovery: per-source health tracking; stale-cache fallback when crashed
- Sprint278IntegrationSummary: poly-parse (C++ + Haskell) end-to-end integration

26/26 tests passing (5+5+5+5+6). All new headers well under 600 lines.
LoRA recorded: sprint278-2026-03-01 (9 calls, 6437 tokens).

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

editor/src/LSPCapabilityNegotiator.h

@@ -0,0 +1,58 @@
+#pragma once
+#include <map>
+#include <set>
+#include <string>
+#include <vector>
+
+namespace whetstone {
+
+class LSPCapabilityNegotiator {
+public:
+    void registerSource(const std::string& source,
+                        const std::vector<std::string>& caps) {
+        caps_[source] = {caps.begin(), caps.end()};
+    }
+
+    bool supports(const std::string& source,
+                  const std::string& capability) const {
+        auto it = caps_.find(source);
+        if (it == caps_.end()) return false;
+        return it->second.count(capability) > 0;
+    }
+
+    // Returns true if the source supports the capability required by this method.
+    // Unknown methods pass through (return true).
+    bool shouldRoute(const std::string& source,
+                     const std::string& method) const {
+        auto it = methodToCap_.find(method);
+        if (it == methodToCap_.end()) return true;  // unknown method: pass through
+        return supports(source, it->second);
+    }
+
+    std::vector<std::string> capabilitiesFor(const std::string& source) const {
+        auto it = caps_.find(source);
+        if (it == caps_.end()) return {};
+        return {it->second.begin(), it->second.end()};
+    }
+
+    std::vector<std::string> sourcesSupporting(const std::string& cap) const {
+        std::vector<std::string> result;
+        for (const auto& [src, capSet] : caps_)
+            if (capSet.count(cap)) result.push_back(src);
+        return result;
+    }
+
+private:
+    std::map<std::string, std::set<std::string>> caps_;
+
+    const std::map<std::string, std::string> methodToCap_ = {
+        {"textDocument/hover",       "hoverProvider"},
+        {"textDocument/definition",  "definitionProvider"},
+        {"textDocument/references",  "referencesProvider"},
+        {"textDocument/completion",  "completionProvider"},
+        {"textDocument/rename",      "renameProvider"},
+        {"textDocument/formatting",  "documentFormattingProvider"},
+    };
+};
+
+} // namespace whetstone