diff --git a/goodwe_controller/test_control.py b/goodwe_controller/test_control.py
index ec1943b..9dd7458 100644
--- a/goodwe_controller/test_control.py
+++ b/goodwe_controller/test_control.py
@@ -24,6 +24,63 @@ def check(name, cond):
         fails.append(name)
 
 
+# ---------------------------------------------------------------------------
+# COVERAGE AUDIT - measured, not executed. Read this before adding a mechanism.
+#
+# THE INVARIANT: every mechanism in compute() must be noticed by AT LEAST TWO
+# checks when it is deleted. If you add a mechanism to compute(), re-run the
+# audit and add it to the table. If a figure here drops, a check has started
+# passing for a reason other than the one it names.
+#
+# THE TECHNIQUE, because there is no script to run: replace one mechanism in
+# control.py with a no-op, run this file, count the failures, restore. That is
+# the converse of the usual mutation - not "does a wrong value fail?" but "does
+# anyone notice when the mechanism is GONE?". It is kept as a comment rather
+# than as tooling on purpose: the only cheap way to automate it is to key on
+# source lines, which goes stale silently, and a green audit that has quietly
+# stopped testing anything is precisely the failure this ticket exists to fix.
+# A comment cannot go stale-green, because it never claims to be running.
+#
+# Measured at 389d9ec. Numbers are the lead's independent reproduction.
+#
+#   mechanism in compute()                     checks that fail when deleted
+#   ------------------------------------------ -----------------------------
+#   integrator freeze (AC 3)                                               2
+#   integrator clamp (AC 1)                                                6
+#   integrator bound follows max_w                                         4
+#   output clamp                                                           3
+#   slew limit                                                             4
+#   output freeze                                                          2
+#   deadband                                                               5
+#   quantisation                                                           2
+#   saturation detector, `saturated_now = False`                          11
+#   saturation duration (AC 2), fires instantly                            2
+#   sat counter reset on a good cycle                                      6
+#   target_grid_w bias                                                     3
+#   i_w=None seeding from prev_w                                           6
+#
+# The detector figure is for the `saturated_now = False` form specifically;
+# disabling it further down as `frozen = False` is a weaker mutation and gives
+# 10. Reproduce the same form or the number will not match.
+#
+# ⚠️ IT HAS FOUND A DEAD MECHANISM TWICE, BOTH THE SAME WAY: a clamp standing in
+# for the mechanism under test. Deleting the integrator freeze once failed
+# NOTHING, because the fixtures sat at max_w 2000 and the integrator bound
+# truncated a wound value back to exactly 2000 - the assertion passed on the
+# clamp. The output clamp was masked the same way by the integrator bound.
+# Hence: A FIXTURE MUST SIT CLEAR OF EVERY RAIL IT IS NOT TESTING. Where a test
+# names one mechanism, make that mechanism the binding one (see TCLAMP and TF).
+#
+# ⚠️ RUN MUTATIONS WITH `python -B` AND CLEAR app/__pycache__. CPython
+# invalidates a .pyc on (source mtime in whole seconds, source size), so a
+# same-second rewrite that also preserves the file size reuses stale bytecode
+# and the suite reports on code you are no longer running. It under-reported one
+# mutation here as 2 where the true figure is 6. The error is one-directional -
+# stale bytecode can only under-report - so every figure above is a lower bound
+# at worst, and the two zeros ever recorded were both confirmed by fixing them
+# and watching the count rise, which a caching artefact cannot do.
+# ---------------------------------------------------------------------------
+
 print("control law")
 
 # Deadband: inside meter noise, hold exactly - do not drift.