feat: add grep_engine_calls (S2 Phase 1)

Greps engine.<namespace>.<func> patterns from Lua source via Lua-pattern.
Returns unique sorted array per spec section 4.2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

init.lua

@@ -61,4 +61,17 @@ function M.diff_surface(surface, readme)
     return { missing_docs = missing, stale_docs = stale }
 end
 
+-- Extracts unique engine.<namespace>.<func> calls from Lua source.
+-- Returns: array of unique strings, sorted.
+function M.grep_engine_calls(source_string)
+    local seen = {}
+    for call in string.gmatch(source_string, "(engine%.[_%w]+%.[_%w]+)") do
+        seen[call] = true
+    end
+    local out = {}
+    for call in pairs(seen) do table.insert(out, call) end
+    table.sort(out)
+    return out
+end
+
 return M