Evaluate php-semver-checker integration for release-compatible API diff checks

[coisa]

Problem

dependencies and release tooling do not evaluate API-level semver compatibility automatically.

tomzx/php-semver-checker could provide objective compatibility checks between current and next states.

Proposal

Evaluate whether php-semver-checker integration provides real value in the changelog/release flow.

If adopted:

• Add a dedicated changelog-supporting command or optional CI check that compares API compatibility.
• Gate it behind explicit workflow input/flag so normal development is not blocked by noisy compatibility deltas.
• Emit machine-readable output when run in automation.

Goals

• Provide evidence for backward-incompatible API changes before tag promotion.
• Keep release decisions more deterministic for packages exposing public APIs.
• Feed release notes with actionable compatibility context.

Expected Benefits

• Reduced accidental major releases caused by unnoticed BC breaks.
• Better support for changelog + release automation in libraries with public surfaces.

Why Not (if skipped)

• Overhead for purely internal apps that do not publish a stable public API.
• False-positive scenarios may require nuanced configuration and maintenance.

Non-goals

• Enforcing semantic version increments automatically today.
• Hard-gating every consumer repository by default.

Acceptance Criteria

• A concrete decision is documented: adopt, defer, or reject integration.
• If adopted, include command defaults, flags, and exclusion strategy.
• If rejected, capture rationale against complexity/false-positive risk.

Architectural / Isolation Criteria

• MUST: Compatibility analysis logic is optional and isolated from standard tests and dependencies flows.
• MUST: Release automation changes remain explicit and opt-in until confidence is proven.