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

third_party/v8/README.md

@@ -0,0 +1,15 @@
+# `rusty_v8` Artifact Placeholder
+
+The Bazel label `//third_party/v8:rusty_v8_archive` currently resolves to this placeholder file.
+
+Replace the filegroup target in `third_party/v8/BUILD.bazel` with the real musl-built
+`librusty_v8` archive before enabling the `codex-v8-poc` crate's `rusty_v8` feature for musl
+targets.
+
+Expected artifact examples:
+
+- `librusty_v8_release_x86_64-unknown-linux-musl.a`
+- `librusty_v8_release_aarch64-unknown-linux-musl.a`
+
+Non-musl Bazel builds intentionally do not receive `RUSTY_V8_ARCHIVE`; they fall back to
+`rusty_v8`'s default build-script behavior instead.