Add PHP conformance runner

[ranael-garem]

Adds the PHP conformance runner (conformance/run.php) — the third SDK to join the cross-SDK behavioral-parity suite alongside TypeScript and Python.

The runner reads one scenario JSON on stdin, drives the real SDK entrypoints, and prints a normalized decision on stdout, byte-identical in shape to the TS/Python runners. All four surfaces are covered:

• verify — drives LicenseTokenVerifier->verify() directly (the public SupertabConnect::verify() returns only a human string; the suite needs the LicenseTokenInvalidReason enum value).
• enforce — SupertabConnect::handleRequest() via a RequestContext built from the scenario headers; emits action / status / the four RSL headers.
• customer-match — LicenseXmlParser::parseContentElements → ContentMatcher::findBestMatch.
• customer-obtain — SupertabConnect::obtainLicenseToken (success → mint, throw → error).

Notes:

• The file lives outside src/ and tests/, so phpcs/phpunit don't scan it — it can't affect this repo's own CI gate. It's exercised by the conformance orchestrator (a separate repo).
• Runs against a fixed mock origin at localhost:9999; base URLs are injected, no SDK network behavior changes.

Paired with the conformance-repo PR that registers the php runner and records its divergences. A dedicated CI job that runs the corpus on every PR to this repo is a follow-up (mirrors the Python SDK's setup).