chore: 删除无用的 mod_add 模块

mod_add_sync 未被 mlkem_top 实例化（模加在各 leaf 算子内部完成），整个
sync_rtl/mod_add 目录（RTL + TB + 向量）已无用。同时:
- 更新 mlkem_top.v 顶部 leaf 模块清单注释（去掉 mod_add_sync，并修正为
  '共享 keccak_core' 的现状）
- run_tb.sh 用法示例改用 ntt 模块

KeyGen KAT 烟囱测试通过。

sync_rtl/top/mlkem_top.v

@@ -16,9 +16,9 @@
 // Built incrementally and verified stage-by-stage against reference golden
 // vectors (test_framework/modules/mlkem_keygen/golden) and NIST KAT.
 //
-// Uses independent (verified) leaf modules, each with its own keccak_core:
-// sha3_top, sample_ntt_sync, sample_cbd_sync, ntt_core, poly_mul_sync,
-// mod_add_sync. No shared-keccak arbiter.
+// Uses independent (verified) leaf modules over a shared keccak_core:
+// sha3_top_shared, sample_ntt_sync_shared, sample_cbd_sync_shared, ntt_core,
+// poly_mul_sync, comp_decomp_sync.
 
 `include "sync_rtl/common/defines.vh"