Add Laravel corpus wall-clock benchmark + CI

[TomasVotruba]

Adds a corpus benchmark similar to the one in the php-parser-speed-comparison repo, self-contained here.

What

• benchmark/ — a small package main that walks a directory, parses every .php file with this parser, and prints parsed N files in M ms. Uses only stdlib (no new dependencies); broken fixtures are skipped since parse errors are ignored (the parse work is what's timed).
• make bench-corpus DIR=./laravel — local runner.
• .github/workflows/benchmark.yaml — clones a full Laravel framework checkout (src/ + Composer vendor/), builds the benchmark, runs it 5 times, and reports the average wall-clock and peak memory to the run Summary. Triggers on push, PR, and every 12h.
• README gains a Benchmark section.

Notes

• The corpus prep (clone + composer install + prune tests dirs) mirrors the comparison repo, so numbers are comparable.
• Verified locally: 2966 files, ~1215 ms; golangci-lint clean, gofmt -s clean, go vet clean.