Add re-runnable router benchmark vs FastRoute and Symfony Routing

[tylersriver]

Closes #7

What

Adds a re-runnable benchmark comparing ruta against nikic/fast-route and symfony/routing (including Symfony's compiled matcher) under various configurations, plus the measured results and an evaluation.

• benchmark/ — standalone composer project so the comparison libraries never become dependencies of ruta itself (ruta is pulled in through a path repository). Run with cd benchmark && composer install && php run.php; flags for route counts, shapes, subjects, iterations and repeats.
• Subjects: ruta, ruta (cached) via Ruta\cachedRouter(), FastRoute simple + cached dispatcher, Symfony UrlMatcher + CompiledUrlMatcher.
• Methodology: deterministically generated route sets in two shapes (REST-style mixed static/dynamic, purely static) at 60/600/1200 routes; measures setup cost and per-dispatch time for static hits, dynamic hits, deep dynamic hits and misses (median of 7 repeats × 20k dispatches). Every subject is verified to actually match/miss each scenario before timing.
• .github/workflows/benchmark.yml — manually triggerable workflow that prints the result tables to the job summary.
• benchmark/README.md — methodology, fairness notes, full results and interpretation.

Headline results (PHP 8.4, opcache off)

• ruta's tree lookup scales with URI depth, not route count: ~1.2 µs → ~3 µs per dispatch going from 60 to 1200 routes; misses stay ~1 µs flat on static sets.
• FastRoute owns static hits (~0.2 µs at any size), but from 600 REST routes up ruta matches/beats it on dynamic hits and is 3–4× faster on misses.
• Symfony's non-compiled matcher degrades linearly (100+ µs/op at 1200 routes); its compiled matcher is the strongest all-rounder at scale.
• ruta has the cheapest cold registration (1.5 ms for 1200 routes vs 9.1 ms FastRoute), and cachedRouter() cuts setup ~3× at 600+ routes.

Note: benchmark/composer.lock is intentionally git-ignored — a path-repository lock pins ruta to the branch name it was generated on and would break composer install after merge.

https://claude.ai/code/session_01HFuiFYMtQfuPKMX4rCifzu

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