test: assertions for grep_engine_calls (S2 Phase 1)

5 assertions: 3 unique calls detected, count matches, no-engine-calls
edge returns empty.

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

init.lua

@@ -80,6 +80,31 @@ Stuff.
         engine.test.equals(#diff3.stale_docs, 2, "diff_surface: 2 stale docs")
     end
 
+    -- ===== grep_engine_calls =====
+    do
+        local src = [[
+local input = require("lib-core.input")
+function update(ctx, dt)
+    if engine.input.is_action_down("foo") then
+        engine.render.text("hi")
+    end
+    engine.window.size()
+end
+]]
+        local calls = api.grep_engine_calls(src)
+        -- Order not guaranteed; check set membership
+        local set = {}
+        for _, c in ipairs(calls) do set[c] = true end
+        engine.test.assert(set["engine.input.is_action_down"], "grep_engine_calls: engine.input.is_action_down")
+        engine.test.assert(set["engine.render.text"],          "grep_engine_calls: engine.render.text")
+        engine.test.assert(set["engine.window.size"],          "grep_engine_calls: engine.window.size")
+        engine.test.equals(#calls, 3, "grep_engine_calls: 3 unique calls")
+
+        -- Edge: no engine calls
+        local empty = api.grep_engine_calls("local M = {}\nreturn M")
+        engine.test.equals(#empty, 0, "grep_engine_calls: empty -> 0")
+    end
+
     engine.exit(engine.test.failures())
 end