feat: add generate_topology_block (S2 Phase 1)

Builds mermaid graph-LR source from manifest.deps + engine-calls.
Asset-libs (no deps, no engine) produce self-node-only output.
Per spec section 4.2.

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

init.lua

@@ -74,4 +74,28 @@ function M.grep_engine_calls(source_string)
     return out
 end
 
+-- Generates mermaid topology-block from manifest + engine_calls.
+-- engine_calls: array from grep_engine_calls (presence determines engine-node).
+-- Returns: mermaid-source-string (no markers - caller wraps).
+function M.generate_topology_block(manifest, engine_calls)
+    local lines = {"graph LR"}
+    local self_node = string.format('  this["%s"]', manifest.id)
+    table.insert(lines, self_node)
+
+    if manifest.deps then
+        for _, dep in ipairs(manifest.deps) do
+            local dep_var = string.gsub(dep.id, "[%-%.]", "_")
+            table.insert(lines, string.format('  %s["%s"]', dep_var, dep.id))
+            table.insert(lines, string.format('  this --> %s', dep_var))
+        end
+    end
+
+    if engine_calls and #engine_calls > 0 then
+        table.insert(lines, '  engine["engine.*"]')
+        table.insert(lines, '  this --> engine')
+    end
+
+    return table.concat(lines, "\n")
+end
+
 return M