Shard children now strip KILO_HTTPAPI_EXERCISE_* so a pinned database
or global root cannot make concurrent shards collide on shared state.
The scenario retry gains a second, longer backoff (1s then 3s): sharded
children run fewer scenarios each, so early scenarios can land before
agent projections warm and one second is not always enough.
Completes #12999 (with the fixture change in the previous commit).
Adds --shard <i>/<n> (round-robin scenario subset) and --shards <n>
(spawns n child processes, each with its own PID-keyed database and
global root, tagged output, combined exit code). test:httpapi runs the
effect pass with --shards 2.
Route coverage (missing/extra) is still computed against the full
scenario list in every child, so --fail-on-missing stays correct.
Local effect-mode wall clock: 4m11s -> 2m39s (6m37s before this PR).
315/315 scenarios pass across both shards.
Yury Zialionka