Add Behat support

[hhamon]

Framework-agnostic core (src/Browser/)

• BrowserOptions (readonly value object) — env-var-driven configuration
• BrowserRegistry — replaces the static state previously held by LegacyExtension
• KernelBooter interface + Test/PhpUnitKernelBooter (closure-fed, captures the trait's privileged scope)
• BrowserFactory — builds KernelBrowser / PantherBrowser from a booter
• Artifact/{ArtifactSink, ArtifactCollector, EchoArtifactSink, FailureType} — the lifecycle/artifact-capture logic extracted out of LegacyExtension

PHPUnit Decoupling

• HasBrowser trait: now ~95 lines, delegates to BrowserFactory with closures granting protected-static access
• LegacyExtension / BootstrappedExtension: thin adapters forwarding events to ArtifactCollector
• BrowserExtension: unchanged externally — same phpunit.xml.dist snippet works

Behat Bridge

• BrowserExtension — Behat ServiceContainer Extension wiring everything
• Context/{BrowserAware, BrowserAwareTrait} — drop-in analog of HasBrowser
• Initializer/BrowserContextInitializer — injects services via Behat's standard DI
• EventListener/ArtifactListener — Suite + Scenario events drive ArtifactCollector
• Output/BehatOutputArtifactSink — STDOUT-direct summary writer
• Kernel/{KernelBooter implementations} — StandaloneKernelBooter (env-var) and SymfonyExtensionKernelBooter (FoB), selected at compile time via class_exists; PantherClientFactory adapter to bypass Panther's protected statics

Composer Update

• composer.json: behat/behat ^3.13|^4.0@dev added to require-dev; both behat and FoB extension listed in suggest
• tests/Behat/{behat.dist.php, behat.yml.dist, Context/BrowserContext.php, features/browser.feature} — exercises the bridge end-to-end against the existing fixture kernel
• README.md: new "Using with Behat" section with context + config examples

Known caveat documented in the plan: friends-of-behat/symfony-extension doesn't yet support Symfony 8, so I could only end-to-end verify the StandaloneKernelBooter path. The SymfonyExtensionKernelBooter is code-complete and PHPStan-clean, but will need either a downgraded composer profile or a FoB release that supports Symfony 8 to verify in CI. The class is wired conditionally so it doesn't break consumers without FoB installed.

Co-Authored with Claude Code

[hhamon]

@kbond I've tested this branch on a personal private project and it works :)

Edit: I'ven't tested with Panther yet.

[kbond]

Awesome @hhamon, thanks for working on this. I've had a quick discussion with @nikophil about it. He uses Behat in his day-to-day so he'll have a look at this when he has some time.