docs(gfql): narrow #1046 post-#1262 child lanes with non-blocking contract alignment - #1347

Summary

• add a durable [FEA] GFQL native type system: schemas, inference, validation, and Arrow representation #1046 execution-map section to ai/docs/gfql/README.md
• make A/B/C child lanes explicit and add lane D for API contract alignment
• document non-blocking policy: D runs in parallel and does not gate A/B/C merges

Why

Post-#1262, A/B/C existed but cross-lane API contract alignment was implicit. This change narrows scope and sequencing so follow-on work is executable without ambiguity or accidental blocking.

Issue graph updates

• new child lane D: GFQL type system follow-on D: API contract alignment matrix + compatibility policy (non-blocking to A/B/C) #1345
• coordination comments posted on [FEA] GFQL native type system: schemas, inference, validation, and Arrow representation #1046, Meta: GFQL compiler/planner post-M4 execution DAG (active wave) #1259, GFQL type system follow-on A: public declarative schema model + stable exports #1337, GFQL type system follow-on B: schema inference API + typed topology extraction #1338, GFQL type system follow-on C: public schema-Arrow APIs + plottable boundary enforcement #1339

Validation

• ./bin/check_docs_latex_unicode.sh

Relates to: #1046