docs(observability-map): correct a mutation's what text and tag a README fence

dead-if-empty-string said append where prependToEveryCatch splices at the head
of the clause. That string is printed in the corpus table and in the Vitest
test name, so it described the wrong position.

The report-output block in the README had no language tag (markdownlint
MD040).

Author: 1stvamp

internal-packages/observability-map/README.md

@@ -119,7 +119,7 @@ rather than celebrating.
 The check list describes a composite the number mostly is not, so the report discloses the
 shape instead of hiding it behind a weight. Today:
 
-```
+```text
 CHECKS
   error-classification  166 applicable,  94 pass,   0 sole, global without it 10
   auth-boundary          62 applicable,  59 pass,   0 sole, global without it 15

internal-packages/observability-map/src/mutations.ts

@@ -633,7 +633,7 @@ export const MUTATIONS: Mutation[] = [
   prependToEveryCatch(
     "dead-if-empty-string",
     "preserving",
-    'append if ("") { throw e; } to every catch',
+    'splice if ("") { throw e; } into every catch',
     (e) => `if ("") { throw ${e}; }`
   ),
   prependToEveryCatch(