feat: init mlkem project with Verilator test framework

- sync_rtl/common/: skid_buffer, pipeline_reg, defines (valid/ready)
- sync_rtl/mod_add/: modular adder example with Verilator C++ TB
- test_framework/: Python-driven Verilator compile/sim/compare pipeline
- test_framework/modules/mod_add/: 50-vector test plan, full鏈路 PASS
- .trellis/spec/: RTL and test_framework conventions documented

.trellis/spec/frontend/directory-structure.md

@@ -0,0 +1,54 @@
+# Directory Structure
+
+> How frontend code is organized in this project.
+
+---
+
+## Overview
+
+<!--
+Document your project's frontend directory structure here.
+
+Questions to answer:
+- Where do components live?
+- How are features/modules organized?
+- Where are shared utilities?
+- How are assets organized?
+-->
+
+(To be filled by the team)
+
+---
+
+## Directory Layout
+
+```
+<!-- Replace with your actual structure -->
+src/
+├── ...
+└── ...
+```
+
+---
+
+## Module Organization
+
+<!-- How should new features be organized? -->
+
+(To be filled by the team)
+
+---
+
+## Naming Conventions
+
+<!-- File and folder naming rules -->
+
+(To be filled by the team)
+
+---
+
+## Examples
+
+<!-- Link to well-organized modules as examples -->
+
+(To be filled by the team)