nightly LAB lane: cargo semver-checks vs 0.71.4 as a heavy-lane receipt (not on PR/merge-queue) #6278

NOAH 15:10Z item 7 (via aprender-b9). Run cargo semver-checks --workspace --baseline-version 0.71.4 nightly on idle machines as a LAB receipt: per-crate CLEAN/BREAKING/not_measured, one ticket with one owner on red, never in a PR, merge-queue or release job (GATE/LAB rule C324). Crates with no buildable rustdoc JSON report not_measured (see #6273). Red or unmeasured 7 days stops the lane until its owner restores it.

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