Follow-up: `maybe_open_context()` `_Cache` cleanup from PR #436

[goodboy]

Follow-up from moc_revert_to_fix (PR #436)

Tracking items identified during the moc_revert_to_fix PR.

• (3f198bc8) Clean up commented-out debug hooks in _Cache
tractor.trionics._mngrs has several await tractor.pause(shield=True) calls left from the diagnostic session
  — remove before merging to main.

• Bundle per-ctx_key state into a CacheEntry dataclass The
  current impl uses parallel dicts for lock, users,
  no_more_users, and resource refs — a single dataclass per
  ctx_key would reduce bookkeeping complexity.

• Assert tn parentage in maybe_open_context() There's a # TODO, assert tn is eventual parent of this task! in the nursery
  setup block that should be enforced to catch misuse.

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(this tracking issue was generated by claude-code)