Prove every ext-state write by reading the key back, so no guard is constant-true

SetProjExtState's return describes the whole extname. The persist and the instrument's publish now re-read their key; both refusals can finally fire.

src/shell/instrument/instrument_bake.cpp

@@ -186,8 +186,13 @@ BakeChainResult runBake(ReaSamplerProcessor& processor) {
 
     const std::string key = bakeKeyFor(instanceGuid);
     RequestKeyGuard keyGuard(bridge, key);
+    // The bridge PROVES this by reading the key back, so a false here is real: the
+    // extension would find nothing to land. It does not say which of the three ways failed,
+    // so neither does this sentence.
     if (!bridge.writeBakeExtState(key, wire::encodeBakeRequest(request)))
-        return fail("could not publish the bake request");
+        return fail("the bake request under " + key +
+                    " could not be confirmed -- it was either never written or did not "
+                    "read back as written, so the extension has nothing to land");
 
     // Synchronous: the extension's landing runs to completion inside this call and writes
     // its outcome back over the same key before returning.