[ty] Share parameter lists with `Arc` (#25735)
## Summary
`Signature` values are cloned throughout callable binding,
deduplication, partial application, and specialization. Because
`Parameters` stored its parameter vector inline, each clone copied the
complete parameter list and retained another allocation.
This change moves the exact-sized parameter slice and its representation
kind into a shared `ParametersData` allocation. Cloning a signature now
clones one `Arc`; the uncommon implicit `self` or `cls` annotation
update uses copy-on-write.
On Home Assistant, this reduced retained memory from 3,230,014,380 bytes
to 3,212,250,858 bytes, saving 17,763,522 bytes (0.55%) without
diagnostic changes.
