A presigned read of THIS review repo's latest index AST-cache bundle, so the review worker can pre-warm its HEAD extraction and re-parse only the PR's changed files (issue #2, Phase 2 -- cold-identical head, extraction cost ∝ the delta).

Same auth as /base-graph and /peer-graphs -- a job token bound to this {job_id} plus an active lease; tenant + repo read from the AUTHENTICATED run. Dark by default behind review_incremental_head_enabled (404 until armed -> the worker cold-builds the head, today's behaviour). A repo with no promoted build surfaces the usual graph-absence error, and a missing bundle object is a download-time 404 -- both are clean cold-build fallbacks, never a failure.