fix(run-engine): remember a drained ck variant's virtual time

A concurrency-key variant whose message zset drained was removed from ckVtime,
throwing away the virtual-time tag it had accumulated. Its next enqueue
re-registered it at the floor, so it came back with full credit. A variant
holding a persistent backlog keeps advancing its tag instead, so it lost every
pass-1 slot to variants that drain and reset each call, and pass 2 could not
help once the batch was already full. Measured on a backlogged variant against
five trickle variants: 1 serve out of 600 with the flag on, against 120 with it
off, inverting the fairness the feature exists to provide.

A parked tag now survives the drain in a sibling :ckVtimeIdle zset, and every
registration path (enqueue, enqueue-with-ttl, nack, and the gated-variant
branch) starts the variant at max(floor, idleTag) rather than at the floor.
The out-of-band drains (ack, dead-letter, TTL expiry) park the tag too. Entries
at or below the floor confer nothing, so a single ZREMRANGEBYSCORE per serving
call reaps them and bounds the set.

Deriving the floor differently was tried first and rejected by measurement: any
variant with a tag that never advances, which includes any concurrency-gated
key, pins the minimum and defeats it.

Backlogged variant now lands on its round-robin share in every shape, including
with a pinned-low gated or future-headed variant present, and with more trickle
variants than batch slots. Op-count overhead goes from 587 to 641 against a
budget of 900. The flag-off Lua is still byte-identical: 7 vtime command
variants changed, the other 31 commands hash the same as HEAD.

Author: 1stvamp