ci: add informational downstream compatibility

[leoromanovsky]

Motivation

Fixture pull requests need early evidence from the tracer implementations without assuming that either the SDK or the shared expectation is automatically correct. PR #23 demonstrated the fan-out while stacked on the regex proposal; this PR establishes the same feedback loop directly from main and independently of regex conformance.

Changes

• Add blocking static validation for ufc-config.json and every evaluation-cases JSON file, including duplicate keys, required envelope fields, map-key consistency, and unknown flag references.
• Run canonical base/head evaluation tests in Go, Java, JavaScript, Python, Ruby, .NET, PHP, and libdatadog.
• Classify each consumer as compatible, new regression, improvement, or existing drift and upload both logs.
• Document the local validation command and the different merge policies for static and downstream checks.

👉 Example JSON fixture failure: #25

Decisions

• Base this PR directly on main; it includes no regex contract or fixture-data changes.
• Keep downstream compatibility advisory and allowed to fail. These jobs must not be configured as required checks.
• Keep static fixture validation blocking because malformed fixture files are unambiguously repository errors.
• Do not automatically update submodule pins, open issues, or decide whether a mismatch belongs to the consumer or fixture.
• Continue invoking consumer-owned test entrypoints from purpose-built branches until those entrypoints land on the repositories default branches.

Validation

• python3 ci/validate-fixtures.py: 258 cases across 33 files pass.
• Duplicate-key and missing-field negative paths reject invalid input.
• bash -n and shellcheck pass for the runner.
• actionlint v1.7.7 and YAML parsing pass for both workflows.
• A real Go base/head run classifies current main as compatible.
• A synthetic failing consumer classifies existing drift and exits successfully, proving the compatibility result is informational.

[btthomas]

#25

Nice! 🚀