Setup a new v8-poc package

This is a new package that depends on rusty-v8 for embedding a v8
engine.  This builds and works on non-musl platforms now because
rusty-v8 provides versions for that.  But we mocked out the loading of
a musl built library.

TODO: iterate on providing the musl built version for musl-linux builds.
TODO: create a release workflow for publishing the artifact that rusty-v8 can link against

codex-rs/v8-poc/src/lib.rs

@@ -0,0 +1,72 @@
+//! Bazel-wired proof-of-concept crate reserved for future V8 experiments.
+
+/// Returns the Bazel label for this proof-of-concept crate.
+#[must_use]
+pub fn bazel_target() -> &'static str {
+    "//codex-rs/v8-poc:v8-poc"
+}
+
+/// Returns the embedded V8 version when the optional `rusty_v8` feature is enabled.
+#[cfg(feature = "rusty_v8")]
+#[must_use]
+pub fn embedded_v8_version() -> &'static str {
+    v8::V8::get_version()
+}
+
+#[cfg(test)]
+mod tests {
+    use pretty_assertions::assert_eq;
+
+    use super::bazel_target;
+    #[cfg(feature = "rusty_v8")]
+    use std::sync::Once;
+
+    #[cfg(feature = "rusty_v8")]
+    fn initialize_v8() {
+        static INIT: Once = Once::new();
+
+        INIT.call_once(|| {
+            v8::V8::initialize_platform(v8::new_default_platform(0, false).make_shared());
+            v8::V8::initialize();
+        });
+    }
+
+    #[cfg(feature = "rusty_v8")]
+    fn evaluate_expression(expression: &str) -> String {
+        initialize_v8();
+
+        let isolate = &mut v8::Isolate::new(Default::default());
+        v8::scope!(let scope, isolate);
+
+        let context = v8::Context::new(scope, Default::default());
+        let scope = &mut v8::ContextScope::new(scope, context);
+        let source = v8::String::new(scope, expression).expect("expression should be valid UTF-8");
+        let script = v8::Script::compile(scope, source, None).expect("expression should compile");
+        let result = script.run(scope).expect("expression should evaluate");
+
+        result.to_rust_string_lossy(scope)
+    }
+
+    #[test]
+    fn exposes_expected_bazel_target() {
+        assert_eq!(bazel_target(), "//codex-rs/v8-poc:v8-poc");
+    }
+
+    #[cfg(feature = "rusty_v8")]
+    #[test]
+    fn exposes_embedded_v8_version() {
+        assert!(!super::embedded_v8_version().is_empty());
+    }
+
+    #[cfg(feature = "rusty_v8")]
+    #[test]
+    fn evaluates_integer_addition() {
+        assert_eq!(evaluate_expression("1 + 2"), "3");
+    }
+
+    #[cfg(feature = "rusty_v8")]
+    #[test]
+    fn evaluates_string_concatenation() {
+        assert_eq!(evaluate_expression("'hello ' + 'world'"), "hello world");
+    }
+}