base: add TaggedPtr raw-store and RefValue set helpers
Add internal helpers to rewrite TaggedPtr values from raw words
and references, plus RefValue convenience methods for storing
TaggedPtr payloads via setindex! and raw updates.
Also adjust codegen tagged-pointer barrier emission to cast
through untracked pointers and extend intrinsics coverage for
the new helper paths.
Co-authored-by: Codex <codex@openai.com>
