feat(gas-estimation): rank on p99 gap + covered-rate column

[Flotapponnier]

Summary

• Leaderboard now ranks on the p99 gap (slot p50 carries the p99 query; ledger_columns relabel honestly). Typical p50 gaps are micro-gwei noise (2e-8 vs 8e-5 gwei), p99 is where gas spikes differentiate oracles.
• New "Covered p50" column via metric_panels: % of time the oracle's prediction sat at or above the realized p50 (inclusion-side risk). Live values: Blocknative ~49% on Ethereum, Etherscan ~1%.
• spec.ts: applyDimensionsToSpec now injects dimension labels into metric_panels queries (was provider-queries only; panels on chain-dimensioned benches silently mixed all chains).
• Full copy pass: disclaimer, seo_intro, methodology, findings, 2 new FAQs.

Depends on

Recording rule ocb:gas_p50_covered:pct_24h (mobula-api PR coming). Column renders "no data" until the rule is deployed; expression already validated against live Prom.