Make lossy downstairs actually lag

[nrempel]

Summary

• change ActiveConnection::do_ready_work so the lossy path explicitly samples skip vs delay decisions (matching the old probabilities) and injects a 50–250 ms sleep when it doesn’t requeue
• context: the lossy delay disappeared in commit d84c259 (“Downstairs task cleanup”, PR Downstairs task cleanup #1313); this restores that behavior
• closes downstairs the --lossy flag no longer works. #1755

Testing

• cargo test
• tools/test_repair.sh -l 3

Snippet from tools/test_repair.sh -l 3 (loop 1 dump showing lag):

0 000-019 ...   T  T  T
1 020-039 ...   T  T  T
2 040-059 ...   T  T  T
3 060-079 ...   F  F  F
4 080-099 ...   T  T  T
...
18 360-379 ...   T  T  T