Derive the `Actor._contexts` key from each `Context` in one
idempotent `Actor._drop_context()` helper instead of reconstructing
the peer UID and CID at every teardown site.
Use the helper for caller-side context exit and preserve a strict
identity assertion when the callee-side RPC task deregisters
itself. Keep channel closure and cancellation shielding with their
existing lifecycle owners.
Caught-during: review remediation
Found-via: staged P2 lifecycle review
Review: PR #481 (opencode)
https://github.com/goodboy/tractor/pull/481#pullrequestreview-4956692120
Prompt-IO: ai/prompt-io/opencode/20260818T193002Z_bf06b4f8_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi