Add docs/RC3_VALIDATION.md from Claro v1.17.26 zip

docs/RC3_VALIDATION.md

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+# Claro RC3 Validation
+
+Validated commands:
+
+```bash
+gcc -std=c99 src/claro.c -O0 -o claro -lm
+./claro --version
+./claro test
+./claro check lessons/01_hello.claro
+./claro check lessons/08_functions.claro
+./claro check tests/checker_fixtures/bad_unknown_command.claro
+```
+
+Expected result:
+
+```text
+Claro RC3
+PASS: 0 failure(s)
+OK
+OK
+```
+
+The bad checker fixture should exit with a non-zero status and print a helpful diagnostic.