feat(agent-sessions): decide every aggregate/filter fact at ingest, the index projects the stamps - #1160

Confidence 4/5 · likely safe to merge
A deliberate behavior change: the span-name/model fallback for a model call now applies only to unknown: vendors, so known-vendor spans with an unrecognized op stop counting as LLM calls.
_{quality 100/100 · no findings · tests covered · risk medium}

The ingest gateway now decides every Agent Sessions fact once per span and stamps it as maple_ai.*; ai_trace_index_mv becomes a projection of those stamps and the detail page reads the same verdicts. Contained and well tested; safe to merge once the ingest deploy ships first.

• facts::stamps writes the gateway's verdicts, names and usage buckets on every stamped AI span
• Migration 0035 recreates ai_trace_index_mv as a projection of maple_ai.* plus error.type and the fingerprint
• classifyAiSpan/spanFailed/spanTokenBuckets take the gateway's verdicts for stamped spans
• gen-ai.test.ts pins MAPLE_AI_STAMP_ATTRS against the Rust key literals

Fixed since the last review

• F1 · Nothing pins the Rust stamp keys to MAPLE_AI_STAMP_ATTRS

_{38f256c · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple to ask about one.}