## Why
V8's Abseil and protobuf's shared Abseil can be built for different C++ runtimes. Give V8's copy a distinct inline namespace so both can link into one binary without interposing incompatible functions.
## What changed
- Move pinned V8 dependency imports and BUILD overlays into `third_party/v8`, and inject those repositories into V8 through a module-extension bridge with no fallback downloads.
- Build V8's Abseil with a V8-specific inline namespace while preserving protobuf's shared Abseil namespace.
- Apply V8's C++ runtime settings in the Abseil, Highway, and simdutf overlays, and remove the shared `rules_cc` patch.
- Document source dependency ownership and the version, checksum, and overlay review procedure for V8 upgrades.
GitOrigin-RevId: 2b8e3deed285559f8dd3324647cb4b81089f2f56