Step 246: mcp_main.cpp — standalone MCP server entry point

Wires HeadlessEditorState + MCPBridge into a standalone whetstone_mcp
binary that any MCP client (Claude Code, Cursor, etc.) can launch over
stdio. No SDL, no ImGui, no OpenGL — headless only. 12/12 tests pass.

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

progress.md

@@ -23,3 +23,25 @@ but with zero GUI dependencies (no ImGui, no SDL, no WebSocket).
 - JSON roundtrip cloning (matching EditorUtils.h) instead of manual deep copy
 - Separate HeadlessAgentState (no AgentMarketplace/WebSocket dependency chain)
 - All files under 600-line architecture limit
+
+### Step 246: mcp_main.cpp — Standalone MCP Server Entry Point
+**Status:** PASS (12/12 tests)
+
+Wires HeadlessEditorState + MCPBridge into a standalone `whetstone_mcp` binary
+that any MCP client (Claude Code, Cursor, etc.) can launch over stdio.
+
+**Files created:**
+- `editor/src/mcp_main.cpp` — Standalone MCP server entry point with CLI arg
+  parsing (--workspace, --language, --verbose), signal handling, resource reader
+  lambda routing whetstone:// URIs, and MCPBridge stdio loop
+- `editor/tests/step246_test.cpp` — 12 test cases: initialize, notifications,
+  tools/list (10+), resources/list (5), prompts/list (4), tools/call pipeline,
+  tools/call get_ast, resources/read ast, resources/read diagnostics, ping,
+  resources/read settings, CLI arg parsing
+
+**Key design decisions:**
+- No SDL, no ImGui, no OpenGL — headless only binary
+- Fixed "mcp-session" session ID (MCP stdio is single-client)
+- Default Refactor role for full MCP access
+- Empty scratch buffer on startup so getAST works immediately
+- All logging to stderr (stdout is the MCP transport)