feat(codegen): add TestRig command

README.md

@@ -107,6 +107,46 @@ antlr4-rust-gen \
 Use multiple roots when a build should emit several independent recognizers in
 one deterministic source-set compilation.
 
+### Run a grammar with TestRig
+
+TestRig can run directly from a grammar-only project. The project does not need
+a `Cargo.toml`, generated Rust sources, or a runtime dependency; it only needs a
+Rust toolchain because TestRig invokes Cargo internally. Install the companion
+command from the codegen package:
+
+```bash
+cargo install antlr-rust-codegen --bin antlr4-rust-testrig
+```
+
+From the grammar project, pass a combined grammar, its parser start rule, and
+zero or more UTF-8 inputs. With no input files the command reads stdin:
+
+```bash
+antlr4-rust-testrig JSON.g4 json --tokens --tree examples/*.json
+echo '{"ok":true}' | antlr4-rust-testrig JSON json --tree
+```
+
+For a lexer grammar, use the special start rule `tokens`. Pair split grammars
+with `--lexer-grammar` and the same import directories used for generation:
+
+```bash
+antlr4-rust-testrig MyGrammarParser.g4 start \
+  --lexer-grammar MyGrammarLexer.g4 \
+  --lib grammar \
+  tests/valid/*.txt
+```
+
+`--trace`, `--diagnostics`, and `--sll` expose the corresponding parser modes.
+Each invocation generates the recognizer and a grammar-specific runner in a
+temporary Cargo package, selects the matching `antlr-rust-runtime` release, and
+removes the generated package afterward. Cargo build artifacts are reused, so
+the first invocation can take longer while dependencies and the runner compile.
+
+TestRig processes every named input and returns a non-zero status when
+generation, temporary runner compilation, input reading, lexing, or parsing
+fails. Recovered lexer and parser syntax errors therefore fail CI even when a
+parse tree was produced.
+
 ### Generated-source/runtime compatibility
 
 Every newly generated lexer and parser contains a compile-time generated-code