core: add TaggedPtr kernel type and layout tracking
Define TaggedPtr{T} in the kernel alongside Ref and Ptr, expose it
as a Core builtin, and add runtime layout tracking for fields that
store TaggedPtr values.
This wires TaggedPtr-aware metadata through datatype layout
construction and propagation, including inline-nested fields, and
updates pointerfree queries to account for tagged slots.
Co-authored-by: Codex <codex@openai.com>
