ci: exercise PR 32 downstream regex conformance

[btthomas]

Important

Evidence only. This draft PR is not intended to merge. It runs the fixture proposed by PR 32 across the existing downstream compatibility workflow.

Motivation

PR 32 changes the portable targeting-regex contract from v2 to v3. This run checks that downstream consumers can load all 77 cases and that the declared Go and Rust engine observations still match production evaluator behavior.

The candidate fixture comes from PR 32 commit 102a45880a1f27047d4dc39feadb79e789ba3ca0. Its checked-in SHA-256 is 7ce5d5f7fb3cb3bb589763c4cb6052493b000c23224aed263255c9c1cda77539.

Changes

• Combine the PR 32 fixture with the downstream workflow from PR 23.
• Run the focused regex test in Go, Java, JavaScript, Python, Ruby, .NET, PHP, and libdatadog.
• Preserve the existing distinction between enforced engine observations and informational native behavior.

Decisions

• Accepted matching behavior is unchanged in v3.
• Native acceptance of a rejected case is not an SDK production bug.
• Failures caused only by a hard-coded v2 version, 75-case count, or old hash belong to the evidence test harness.
• A mismatch in a declared Go or Rust engine observation needs investigation before PR 32 merges.

Checklist

• JSON files are valid.
• The canonical regex validator, negative validator tests, SHA-256 check, and fixture validator pass.
• The workflow script passes bash -n.
• New regex cases are standalone and do not require flag configuration or evaluation cases.