Highlights

• Added the experimental plantree.StructuralSignature API for deterministic, occurrence-ordered comparison of visible relational plan structure.
• The current encoding token is spannerplan.structural_signature.v1alpha1. It includes operator display names, parent link types, recursively typed metadata, predicates, and ordered DAG occurrences while excluding PlanNode IDs and execution statistics.
• Metadata encoding is byte-length framed and key-sorted. Invalid, non-JSON, or forward-unknown protobuf states fail closed instead of producing an ambiguous signature.
• Added ecosystem/matrix.json as the machine-readable source for the ecosystem tables in ECOSYSTEM.md, with validation and rendering checks.
• Added a non-blocking scheduled/manual canary that verifies recorded public consumer refs and their pinned go.mod dependency content without claiming current downstream compatibility.

Stability

This remains an intentionally unstable v0 prerelease. The structural signature is a v1alpha1 experiment, not a stable v1 or cross-version interchange contract; its field set and encoding may change in later alpha releases. It is separate from any future external structured Plantree DTO.

Verification

• go test ./...
• go vet ./...
• golangci-lint v2.12.2 — 0 issues
• go run ./ecosystem/cmd/canary -live — all three recorded public pins passed
• Pull-request CI for the ecosystem matrix/canary and structural signature changes