bootstrap from 2026.9.20.1, and widen an assertion that matched an identifier

2026.9.20.1 is released, mirrored on both ends and resolvable from the index,
so the workspace bootstraps from it.

VERIFIED BEFORE THE PIN MOVED, not after. The published CN artifact reports
its own version and builds openkal-musl's real 24-row [c-abi-absent] manifest;
`.agents/docs/2026-09-20-cenv-interfaces-verify.sh` reports `fails=0` against
it both on this host and inside `xlings subos use v920 --sandbox`, with CN
mirrors, addressing the store path rather than the shim --- `xlings use`
rewrites only the root configuration, so a shim is not evidence of which
binary a sandbox runs. The same script against the published 2026.9.18.3
reports `fails=2`, which is what makes each CHANGE section a measurement
rather than a formality.

The other change is a test. `AFreestandingTargetWithoutASelectionIsRefused`
asserted that the refusal contains the target triple --- an identifier, which
sits in the right place in a sentence that is wrong. Two refusals this wave
added were found malformed exactly that way, by rendering them and reading
them: one had lost a closing parenthesis, and one read as the claim it denies.
This message has a single substitution and no paired delimiters, so it has no
such failure mode; the assertion is widened anyway, because "no failure mode"
is a property of today's wording rather than of the check.

Author: speak-agent

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "2026.9.17.1"
+    "mcpp": "2026.9.20.1"
   }
 }

tests/unit/test_cenv_probe.cpp

@@ -300,6 +300,16 @@ TEST(CenvProbeArgv, AFreestandingTargetWithoutASelectionIsRefused) {
     ASSERT_FALSE(r.has_value())
         << "an argv with no target selection must be refused for a "
            "freestanding target; it measures the build host";
+    // THE CLAUSE, NOT ONLY THE TRIPLE. Two other refusals this wave added were
+    // found malformed by rendering them: one lost a closing parenthesis, and
+    // one read as the claim it denies. Both had tests asserting an identifier
+    // inside the sentence, and an identifier sits in the right place in a
+    // sentence that is wrong. This message has one substitution and no paired
+    // delimiters, so it has no such failure mode -- the assertion is widened
+    // anyway, because that is a property of today's wording rather than of
+    // the check.
+    EXPECT_NE(r.error().find("would have run with no target selection"),
+              std::string::npos) << r.error();
     EXPECT_NE(r.error().find("riscv64-none-elf"), std::string::npos)
         << "the refusal must name the target it was for: " << r.error();
 }