ci: check_shard_count_single_source has no row for the merge_group key/literal (#5820 follow-up) #5821

Found in #5820's quorum (w5032 minor a; same gap in my read). After #5820, the merge-queue shard count lives in three places: the workspace-test-shard-merge_group pins key in ci/sections.yml, the 'merge_group' literal in the ci.yml matrix and in the Σ awk. A typo in the key or the literal fails CLOSED (the Σ-executed check goes red, because the union of the shards that ran != the universe), but no guard row names it, so the first signal is a red merge queue instead of a red guard.

Done when: scripts/check_shard_count_single_source.sh asserts (1) the pins key name exists once, (2) the 'merge_group' literal count in the matrix expression and the Σ awk, each with a planted mutant that turns RED; plus (b) check_shard_union self-test has an M=4-only mutant and (c) shard_union_nextest.sh's merge_group loop has a self-test; (d) ci/sections.yml:29 comment still says TWELVE shards.

Owner: w5032 (after #5820 merges). Class: LAB/guard hardening, not a gate change. Refs #5820, #5819.