refactor(hw/sim): reorganize CAM top-level tests into per-noise-config suites

Split the monolithic test_cam_basic.py into separate test suites
organized by noise configuration (no_noise, write_noise, read_noise),
with shared utilities extracted to tests/top/utils.py.

- Remove test_cam_basic.py; add no_noise/, write_noise/, read_noise/
  test suites with Makefiles that set noise parameters statically
- Extract helpers (reset_dut, write_rows, query_once, collect_topk,
  etc.) into tests/top/utils.py
- Update hw/sim/Makefile with per-config test targets and a
  test-top-all meta-target
- Update scripts/run_cam_correctness.py to build per-directory
  instead of centrally, removing inline parameter overrides
- Add __init__.py for result_serializer and topk_tracker module tests
- Expand docstrings in test_ref_model_noise.py with architectural
  context and test rationale

hw/sim/tests/top/write_noise/Makefile

@@ -0,0 +1,29 @@
+SIM_ROOT := $(abspath ../../..)
+RTL_ROOT := $(abspath $(SIM_ROOT)/../rtl)
+include $(SIM_ROOT)/mk/rtl-sources.mk
+
+TOPLEVEL := cam_top
+COCOTB_TEST_MODULES := tests.top.write_noise.test_write_noise
+VERILOG_SOURCES := $(RTL_CAM_TOP)
+
+# 写入噪声 ~1% 默认速率
+WRITE_NOISE_EN := 1
+WRITE_NOISE_RATE_NUM := 1
+WRITE_NOISE_RATE_DEN := 100
+READ_NOISE_EN := 0
+
+include $(SIM_ROOT)/mk/cocotb-common.mk
+
+# ── 速率变体子目标 ─────────────────────────────────────────────────
+.PHONY: test-zero-rate test-full-rate
+
+test-zero-rate:
+	$(MAKE) -B -f Makefile results.xml WRITE_NOISE_RATE_NUM=0 \
+	  COCOTB_TEST_FILTER=zero_rate_noise
+
+test-full-rate:
+	$(MAKE) -B -f Makefile results.xml WRITE_NOISE_RATE_NUM=1 WRITE_NOISE_RATE_DEN=1 \
+	  COCOTB_TEST_FILTER=full_rate_noise
+
+clean::
+	rm -rf sim_build