benchmark: t0 merge driver follow-ups — snapshot completion sentinel, early manifest-lineage assert

[Xore]

Non-blocking follow-ups from the #3137/#3138 review (F3/F5):

• round7_t0_merge_export.sh: the base snapshot_download stage has no completion sentinel — a partial download leaves the dir present and a re-run skips then fails at convert. Same .merge_done pattern as the merge stage.
• Manifest lineage assert (base_f16_sha != merged_f16_sha) runs in stage 5, after ollama create has already registered the tags — move it before stage 4 so a lineage collision cannot register.

Found in review of #3138; not blockers for that merge.

[Xore]

Orchestrator claimed — coding dispatch (worktree issue-3139 off 09b863e).

[Xore]

PR up: #3157. snapshot dir now uses a .snapshot_done sentinel (mirrors .merge_done, wipes dir on re-entry when marker absent); lineage assert moved to a new stage 3c right after both f16 GGUFs exist, before stage 4 registers any Ollama tag. CI running.

[Xore]

PR #3157 up (both fixes: .snapshot_done sentinel + lineage assert moved before stage 4). CI held on red — backend-service check fails, but root cause is unrelated to this diff: homeserver's /mnt-1 RAID VD went offline tonight (two-drive enclosure fault), which is where the CI buildx cache lives. Filed #3158, not merging until that's resolved and the check re-runs clean.