fix(flags): specify versioned property matching

[marandaneto]

Problem

Local feature flag evaluators need to follow the versioned boolean matching introduced in PostHog/posthog#90694. The shared spec currently requires legacy truthiness matching for every project and does not describe the new response metadata.

Changes

• Define property_matching_version on the definitions response. Missing or version 1 uses released server behavior. Exactly version 2 uses explicit scalar equality and membership in nonempty filter arrays.
• Preserve the special empty-filter behavior and keep is_not as the complement of exact for known properties.
• Require the matching version to stay with the definitions snapshot through caches, refreshes, group conditions, recursive cohorts and flag dependencies.
• Cover version-only changes, older cache entries, failed refreshes and in-flight evaluations in acceptance fixtures.
• Apply and archive the OpenSpec change. This PR contains specs and acceptance fixtures only, not SDK implementations or updated compliance claims.

Related opt-in harness coverage: PostHog/posthog-sdk-test-harness#53.

Validation

• openspec validate --specs --strict: 62 specs passed.
• Parsed and compiled all 60 Gherkin files, covering 718 scenarios.
• Verified the six cases from the backend PR for missing, 1 and 2, with both exact and is_not.
• Final branch autoreview against origin/main: no actionable findings.