refactor(cam): rename read-noise path to read-pass-through and reorganize test module structure

- Rename `read_noise` scenarios and noise_mode to `read_pass_through` across
  run_cam_correctness.py, test_run_cam_correctness.py, and Makefiles
- Update RTL comment in match_engine_pipeline.sv to reflect pass-through behavior
- Move unit-level cocotb tests from `tests.test_*` flat namespace to
  `tests.modules.*` and `tests.top.*` subdirectory layout, matching actual
  Makefile paths (hw/sim/tests/modules/..., hw/sim/tests/top/...)
- Remove redundant dual-noise subtarget from read_noise/Makefile
- Update help text and docs to reflect read-path pass-through semantics
- Add .codegraph to .gitignore

hw/sim/tests/modules/cam_read_noise/test_cam_read_noise.py

@@ -52,7 +52,7 @@ async def read_noise_disabled_forwards_hashes_after_one_stage(dut):
 
 @cocotb.test()
 async def read_noise_enabled_still_forwards_hashes_unmodified(dut):
-    """With READ_NOISE_EN=1, the pass-through still forwards hashes unmodified."""
+    """The read path pass-through forwards hashes unmodified."""
     cocotb.start_soon(Clock(dut.clk, 10, unit="ns").start())
     await reset_read_noise(dut)