Add phpbench benchmarks with CI regression detection

[veewee]

Summary

• Adds phpbench benchmark suite comparing php-soap/encoding against ext-soap (C extension)
• Tests both literal and encoded binding modes, at x1 and x500 item scales
• GitHub Actions workflow runs benchmarks on base + PR branch on the same runner, then diffs with --ref to surface regressions
• Standalone profiling scripts in benchmarks/scripts/ for large payload throughput (50-500MB) and micro-profiling

Benchmark structure

EncodeBench                         DecodeBench
  benchLiteral_x1                     benchLiteral_x1
  benchLiteral_x500                   benchLiteral_x500
  benchEncoded_x1                     benchEncoded_x1
  benchEncoded_x500                   benchEncoded_x500
  benchExtSoapLiteral_x1              benchExtSoapLiteral_x1
  benchExtSoapLiteral_x500            benchExtSoapLiteral_x500
  benchExtSoapEncoded_x1              benchExtSoapEncoded_x1
  benchExtSoapEncoded_x500            benchExtSoapEncoded_x500

Groups: --group=literal, --group=encoded, --group=ext-soap, --group=x1, --group=x500

Usage

# Full suite
vendor/bin/phpbench run benchmarks/ --report=aggregate

# Just encode, literal mode
vendor/bin/phpbench run benchmarks/EncodeBench.php --group=literal --report=aggregate

# Store a run and compare later
vendor/bin/phpbench run benchmarks/ --store --tag=before --report=aggregate
# ... make changes ...
vendor/bin/phpbench run benchmarks/ --ref=before --report=aggregate

Test plan

• vendor/bin/phpbench run benchmarks/ --report=aggregate completes with 16 subjects, 0 errors
• --group=ext-soap filters to ext-soap benchmarks only
• CI workflow runs on this PR and shows comparison output