Regex handling

• Analyze regex patterns for matching complexity and excessive backtracking, including on long and near-matching inputs. Prefer equivalent patterns with less backtracking when available, while preserving matching semantics and target-framework compatibility.
• For fixed patterns on modern targets, use source-generated regexes with explicit match timeouts (GeneratedRegex under NET8_0_OR_GREATER).
• Use conditional compilation to provide a regular Regex with the same pattern and an explicit match timeout for older targets. Do not duplicate regex validation with a manually maintained character scanner.
• Older-runtime regex matching may still time out under load because timeouts use wall-clock time. If consumers encounter this limitation, recommend upgrading to a modern runtime that uses the source-generated implementation.
• Keep shared patterns in constants and reference those constants in validation diagnostics and tests.