Qualify the shared cadence and prepare schedule cutover

[gauravtiwari]

The shared cadence is merged in #42. Live canary qualification exposed failures in prepared-source verification, comparator cache restoration, OBS setup/reporting and concurrent source publication. This change corrects those paths and retains the executed observations and verified evidence archives.

• Verify prepared snapshots against their declared upstream parent, remove REAPI seed metadata before starting bazel-remote, install the required ccache release, and preserve missing OBS cache-hit data as unmeasured.
• Retry only GitHub's exact concurrent-ref conflict; retain ordinary permission failures. The isolated publication rehearsal now passes conditional commits, independent dispatch, reconciliation and simultaneous writers.
• Keep case, variant, CLI channel and lane labels consistent. Collect canonical phase records from bundles that also contain native evidence. Preserve split-archive metadata in the catalog.
• Retain frozen seed/advancement plans and all earlier failed or cancelled cohorts. The qualification review covers 12 cases / 17 rolling targets. All seventeen targets passed both runs. Helix and Zed Nix each passed two fresh samples. All completed observations are collected, with 60 verified qualification/rehearsal archives.
• Prepare, but do not apply, removal of 48 historical cron triggers in 17 repositories. Central activation remains off pending a compatible stable CLI release and the documented drain/cutover procedure.

Validation: 252 tests / 1,992 assertions pass. Case, registry, workflow boundary, report-contract and Actionlint checks pass. The full hosted source rehearsal passed 24 cases, and the fresh dispatch dry run passed 31 targets. See docs/cadence-qualification-2026-10-05.md and docs/cadence-cutover.md for evidence and remaining gates.