shard split guards: typed shard counts escape, nextest union probe covers one crate (follow-up to #5794) #5807

Two guard-strength gaps found by quorum on #5794 @4a37c41882 (receipts agy-5794-4a37c41882.json, claude-lane-5794-4a37c41882.md):

1. check_shard_count_single_source.sh escaped by hand-typed counts: --shard "$SHARD/3" (unbraced), quick_tier_shard.sh "$SHARD" 3, and SHARDS=3 in the aggregator (ci.yml:519).
2. shard_union_nextest.sh probed only aprender-contracts --lib.

Also fixed: duplicated cd line in shard_union_nextest.sh. Not addressed: the inert jobs matrix copy in ci/sections.yml:90, check_shard_union.sh self-test accepting any non-zero exit, and the 3-shard numbers in ci/shard-step-costs.tsv.