Enable V8 sandboxing for source-built builds (#21146)

## Summary

This is the first PR in the V8 in-process sandboxing rollout.

It adds the build-system and Rust feature plumbing needed to support
sandboxed V8 builds, then enables sandboxing by default for the
source-built Bazel V8 path that we control directly. It deliberately
keeps the published `rusty_v8` artifact workflows on their current
non-sandboxed contract so this PR can land and ship independently before
we change any released artifacts.

## Rollout plan

- [x] **PR 1: land sandbox plumbing and default source-built Bazel V8 to
sandboxed mode**

- [ ] **PR 2: publish sandbox-enabled release artifacts and add
compatibility validation**
- Produce sandboxed artifact pairs for every released Cargo target that
does not already use the source-built Bazel path.
- Add CI coverage that consumes those sandboxed artifacts and verifies:
    - `codex-v8-poc` reports sandbox enabled
    - `codex-code-mode` builds/tests against the sandboxed path

- [ ] **PR 3: switch release consumers to sandboxed artifacts by
default**
  - Update released artifact selectors/checksums.
- Enable the Rust `v8_enable_sandbox` feature in the default release
path.
- Make the sandboxed artifact family the normal path for published
builds.

- [ ] **PR 4: remove rollout-only compatibility paths**
- Remove the temporary non-sandbox release compatibility config once the
new default has shipped and baked.
  - Keep the invariant tests permanently.

.github/workflows/rusty-v8-release.yml

@@ -111,6 +111,7 @@ jobs:
             -c
             opt
             "--platforms=@llvm//platforms:${PLATFORM}"
+            --config=v8-release-compat
             "${pair_target}"
             "${extra_targets[@]}"
             --build_metadata=COMMIT_SHA=$(git rev-parse HEAD)
@@ -134,6 +135,7 @@ jobs:
             --platform "${PLATFORM}" \
             --target "${TARGET}" \
             --compilation-mode opt \
+            --bazel-config v8-release-compat \
             --output-dir "dist/${TARGET}"
 
       - name: Upload staged musl artifacts

.github/workflows/v8-canary.yml

@@ -105,6 +105,7 @@ jobs:
           bazel_args=(
             build
             "--platforms=@llvm//platforms:${PLATFORM}"
+            --config=v8-release-compat
             "${pair_target}"
             "${extra_targets[@]}"
             --build_metadata=COMMIT_SHA=$(git rev-parse HEAD)
@@ -127,6 +128,7 @@ jobs:
           python3 .github/scripts/rusty_v8_bazel.py stage-release-pair \
             --platform "${PLATFORM}" \
             --target "${TARGET}" \
+            --bazel-config v8-release-compat \
             --output-dir "dist/${TARGET}"
 
       - name: Upload staged musl artifacts