Epic: Shard split, mg-reuse guards and crate fan-out (FLOW-010 P1/P2) #5961

Children tracked here (REDUCE-1022)

• shard split guards: typed shard counts escape, nextest union probe covers one crate (follow-up to #5794) #5807 — shard split guards: typed shard counts escape, nextest union probe covers one crate (follow-up to ci: split workspace-test-shard 3 -> 12 with one count source #5794) (closed: not planned)
• Review round: infra#1995 receipt-judge runner (agy lane) #5811 — Review round: infra#1995 receipt-judge runner (agy lane) (closed: not planned)
• ci: MG shard jobs queue up to 28 min on 5 mq runners (12 shards + x86-main); shard count vs mq runner ratio #5819 — ci: MG shard jobs queue up to 28 min on 5 mq runners (12 shards + x86-main); shard count vs mq runner ratio (closed: not planned)
• ci: check_shard_count_single_source has no row for the merge_group key/literal (#5820 follow-up) #5821 — ci: check_shard_count_single_source has no row for the merge_group key/literal (ci: merge_group runs 4 workspace-test shards (PR/push stay 12); fat_driver reads -merge_group pins (#5819 option a) #5820 follow-up) (closed: not planned)
• LAB schedule runs (dogfood-nightly, toolchain-ceiling) take mq runners during STOPLINE #5825 — LAB schedule runs (dogfood-nightly, toolchain-ceiling) take mq runners during STOPLINE (closed: not planned)
• Five-whys: crate fan-out — each PR affects ~49.6 of 85 crates; shard-minutes per core edge #5833 — Five-whys: crate fan-out — each PR affects ~49.6 of 85 crates; shard-minutes per core edge (closed: not planned)
• Cross-compile spike: gx10 builds the x86_64 workspace and nextest archive (go/no-go) #5837 — Cross-compile spike: gx10 builds the x86_64 workspace and nextest archive (go/no-go) (closed: not planned)
• ci: fat_driver container exec drops the runner's thread caps #5881 — ci: fat_driver container exec drops the runner's thread caps (closed: not planned)
• mg-reuse shards(): require shard indices exactly 1..M, not just count==M #5895 — mg-reuse shards(): require shard indices exactly 1..M, not just count==M (closed: not planned)
• FLOW-010 P1 follow-up: prove union(shard lists) == live.json for the nextest archive (#5854 operator 13:25Z condition) #5904 — FLOW-010 P1 follow-up: prove union(shard lists) == live.json for the nextest archive (ci(FLOW-010 P1): nextest archive — build the tree-reader set once, shards run from it (squashed; supersedes #5812) #5854 operator 13:25Z condition) (closed: not planned)
• CI: debug-build pv lints the whole contracts/ tree in tests (7x = 617% CPU on intel) #5914 — CI: debug-build pv lints the whole contracts/ tree in tests (7x = 617% CPU on intel) (open)
• CI: git gc --auto burns CPU in fat workspaces on intel runners (set gc.auto=0 + nightly gc) #5917 — CI: git gc --auto burns CPU in fat workspaces on intel runners (set gc.auto=0 + nightly gc) (closed: not planned)