fix: pm bench audit

[Flotapponnier]

Full audit of all 4 PM benches and the harness after the Metaculus add + Betfair remove work.

Breaking fixes

• pm-stats.ts: Add manifold, predictit, smarkets, metaculus to PM_VENUES — without this, those 4 venues had no cohort.venues entry so their product pages returned null for PM context
• pm-venue-context.ts: Add predictit and smarkets to PM_VENUE_META (same issue — product pages returned null immediately)
• pm-rate-limits.yml: Remove metaculus from ramp_added_latency panel exclusion — the harness runs a ramp for Metaculus (RampRates: 5/10/20) so the exclusion was silently dropping real data

Moderate fixes

• pm-api-latency.yml: subtitle "seven" → "eight"; seo_intro venue list now includes PredictIt/Smarkets/Metaculus; methodology endpoint bullet now documents all 8 venues; findings "five venue APIs" → "eight venue APIs"
• pm-rate-limits.yml: seo_description and subtitle updated to 8 venues; methodology endpoint bullet and ramp clamp bullet now cover PredictIt/Smarkets/Metaculus
• pm-ws-latency.yml: subtitle now correctly says Kalshi is "measured, not ranked" (rank_matrix is Polymarket-only per PR fix(pm-ws-latency): remove Kalshi from rank_matrix #1697); methodology Kalshi hostname fixed (external-api-ws → api.elections.kalshi.com/trade-api/ws/v2); Metaculus per_chain_explainer entry added

Minor fixes

• config.go: userAgent contact@mobula.io → contact@openchainbench.com (matches methodology pages)
• pm-api-latency.yml: PredictIt provider queries now include cache!="hit" for consistency with all other venues