docs(zsh-standard): require a removal's test to be able to fail

.github/instructions/zsh-scripting.instructions.md

@@ -315,6 +315,31 @@ and remember that a system `zshenv` may still execute.
 Mark intentionally invalid, legacy, or lint-negative fixtures in repository
 metadata instead of excluding an entire test directory.
 
+### `zsh/test/cover-removed-behavior`
+
+- Level: `required`
+- Profiles: `test-fixture`
+- Minimum Zsh: `null`
+- Basis: `organization-policy`
+- Evidence: `manual-index`
+- Enforcement: `human-review`
+
+When a change removes code on the grounds that it is unnecessary, a test
+offered as evidence for that removal must exercise the behaviour the removed
+code provided. Observe it failing with the code removed and passing with it
+present. A test that passes in both states supports nothing and must not be
+cited as though it does.
+
+Run the candidate test against the unmodified source before removing anything.
+If it passes there, it is not evidence, and either the test or the premise of
+the removal is wrong.
+
+This applies with particular force where a line looks inert: a declaration that
+appears redundant, an assignment whose value seems unused, a mutation that
+resembles a leak. Such a line is exactly the kind that gets removed on
+inspection, and exactly the kind whose loss a nearby test will not notice
+because the test covers the path the line does not serve.
+
 ### `zsh/test/match-production-profile`
 
 - Level: `required`

lib/zsh-standard-policy.json

@@ -310,6 +310,15 @@
       "evidence": ["shell-grammar"],
       "enforcement": ["classifier"]
     },
+    {
+      "id": "zsh/test/cover-removed-behavior",
+      "level": "required",
+      "profiles": ["test-fixture"],
+      "minimum_zsh": null,
+      "basis": "organization-policy",
+      "evidence": ["manual-index"],
+      "enforcement": ["human-review"]
+    },
     {
       "id": "zsh/test/match-production-profile",
       "level": "required",

scripts/test_validate_zsh_standard_policy.py

@@ -1979,7 +1979,7 @@ def test_repair_2_consumer_parser_outputs_match_frozen_golden(self) -> None:
             },
             "parsed_rules": validator._markdown_rules(instruction),
         }
-        self.assertEqual(len(snapshot["rule_blocks"]), 65)
+        self.assertEqual(len(snapshot["rule_blocks"]), 66)
         digest = hashlib.sha256(
             json.dumps(
                 snapshot,
@@ -1991,7 +1991,7 @@ def test_repair_2_consumer_parser_outputs_match_frozen_golden(self) -> None:
 
         self.assertEqual(
             digest,
-            "d841ec864632352bc399bb035780e6805200425a4f14b0b700d3e7a9e8f7c59e",
+            "c2faea55b03018e947b6e1b5fef023c4d50a9474cfc9f4c4f372f68cabf6ba58",
             msg=(
                 "The frozen golden covers the parsed output of every path in "
                 f"{paths}. Editing any of them changes this digest, which is "

scripts/validate-zsh-standard-policy.py

@@ -144,6 +144,7 @@
     "zsh/completion/preserve-trust-boundaries",
     "zsh/test/isolate-environment",
     "zsh/test/declare-negative-fixtures",
+    "zsh/test/cover-removed-behavior",
     "zsh/test/match-production-profile",
     "zsh/options/canonical-spelling",
     "zsh/options/declare-correctness-state",