fix(001): true cross-chain avg with avg by (aggregator)

[Flotapponnier]

Problem

Previous fix removed All chains to hide the bug, but the real root cause was never fixed: prom.scalar takes result[0] from multi-series Prometheus vectors. Series come back alphabetically, so chain="base" always wins and every provider's global p50 was secretly just their Base latency.

Codex (0.8s on Base) appeared as global #1 despite Mobula winning on BNB, Solana, and Robinhood.

Fix

Wrap every p50/p90/p99/mean/success scalar query with avg by (aggregator) (...):

# Before — returns result[0] = Base data by alphabet
quantile_over_time(0.50, head_lag_seconds{aggregator="mobula"}[24h]) * 1000

# After — collapses to a single honest cross-chain mean
avg by (aggregator) (quantile_over_time(0.50, head_lag_seconds{aggregator="mobula"}[24h])) * 1000

When a chain filter is injected (e.g. Base tab selected), avg by (aggregator) collapses the 3 region series instead → still one clean scalar per provider.

After

• All chains tab restored with honest data
• Mobula ~858ms (avg Base 1477 + BNB 1000 + Solana 86 + Robinhood 868) < Codex ~1021ms → correct global leader
• Per-chain tabs unchanged: Codex still correctly shows as drop journal framing #1 on Base
• AT A GLANCE, homepage, search all now show Mobula as global leader