fix(data): cache layer hardening per multi-agent review + ledger columns

[Flotapponnier]

Summary

Multi-agent adversarial review of the data layer surfaced 9 real issues; all fixed:

• Prom client: global 8-slot concurrency cap (cold-start burst was browning out Prom, the root cause of partial leaderboards) + one retry on null percentiles
• Quorum floored at 3 so a legitimately decimated provider field can't draft forever after snapshot expiry
• Snapshot ratchet now compares live coverage (was dead code: results array is padded with unavailable rows, lengths always matched); KV read failure skips the write instead of bypassing the guards; after() keeps the lambda alive until the fire-and-forget write lands
• cellRanks: case-insensitive chain/region label matching; marginals restricted to providers covering every cell of the collapsed dimension (kills Simpson's bias where a single-region provider wins the chain average by omission, and keeps badge ranks consistent with the per-cell wins that earned them)
• Product badges: collapsed claims (global/row/column) require full declared cell coverage, a one-cell degraded matrix can't mint a global drop journal framing #1
• Badge SVG: scope label XML-escaped in attributes; last bare 404 got cache-control
• Also carries the ledger_columns feature (per-bench ledger column relabeling) from the parallel session

Test plan

• tsc, bun test, validate-specs (25 specs) pass
• Staging: bench pages stable through a deploy (no partial boards), badges unchanged for dRPC/Mobula/Jupiter