Map dbt upstream references

[sabino]

Summary

• add a public-safe upstream dbt source reference map for dxt compatibility planning
• document the source-grounded compatibility method in PLAN.md
• identify the next five small M1/M2 slices and the remaining gates before M2 product implementation

Notes

This is a research/planning-only branch. It does not change Zig product runtime behavior. The map treats dbt Core 1.latest as the compatibility contract and dbt Core v2 / Fusion main as an alpha architecture reference.

Validation

• zig build test --cache-dir /tmp/dxt-zig-cache --global-cache-dir /tmp/dxt-zig-global-cache
• pytest -q
• python scripts/check_runtime_boundary.py
• python scripts/check_public_safety.py
• git diff --check
• scoped local-path/secret scan over PLAN.md and .agent/research/dbt-upstream-reference-map.md
• subagent progress audit and upstream reference research
• codex exec read-only review; one source-map finding was fixed before PR