Add performance audit document for ModSecurity v3

[Easton97-Jens]

Motivation

• Provide a concise, code-focused performance audit of libmodsecurity v3 to document hot paths, parsing and rule-engine bottlenecks and to guide optimization efforts.

Description

• Add doc/performance_audit_modsecurity_2026-04-03.md which contains a technical analysis of the request lifecycle, rule-evaluation hot paths (RulesSet::evaluate, RuleWithOperator::evaluate), parsing (URL-encoded/JSON/XML/multipart), regex/operator costs, collection locking behavior and a prioritized list of optimization opportunities; this change only adds documentation and does not modify runtime code.

Testing

• Verified the new file was added and committed and inspected repository state using git status --short, git rev-parse --short HEAD, and git show --stat --oneline -1.

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