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ci: retire Spec Coverage and Engine Split Metric; run the showcase smoke in the Nightly Tiers live tier #51313

Job	Run time
Lint · ESLint	4m 2s
Lint · PM dispatch-gates	6m 27s
Type Check · workspace	17m 48s
Type Check · consumer gates	8m 7s
Lint · CI, release & engine gates	4m 27s
Lint · scoped · query-options erasure	4m 24s
Lint · repo & tooling gates	3m 45s
Lint · scoped · slot lookup & corpus	4m 12s
Type Check · source gates	4m 55s
Lint · docs & code guards	4m 54s
Type Check · debt ledger	5m 41s
Lint · scoped · tooling self-tests	5m 5s
Lint & Repo Gates	5s
TypeScript Type Check	5s
	1h 13m 57s