`chain_ref: u16` selects the preprocessing chain for this tensor: - High bit (`0x8000`) set: an inline chain payload follows the fixed header (before dependencies and per-terminal records). - High bit clear: `chain_ref & 0x7FFF` is a chain registry index.

`chain_ref` encodes the chain association: - `chain_ref & 0x8000 == 0`: the tensor's chain is in the container chain registry at index `chain_ref & 0x7FFF`. `inline_chain` must be `None`. - `chain_ref & 0x8000 != 0`: the chain is stored inline in this record (in `inline_chain`). The low 15 bits are a hint or zero. `terminals` (formerly `planes`) hold the per-output codec records in terminal order; `num_terminals` is derived from `terminals.len()` on write.