Add custom string recognizers

[tinysec]

Summary

• add managed and core-backed StringRecognizer registration, lookup, enumeration, and all stable recognition operations
• marshal the complete BNCustomStringRecognizer callback table with rooted delegate lifetimes and exception containment
• preserve DerivedString locations and CustomStringType handles across native round trips
• correct list, callback-table, and derived-string output ABIs
• validate the full managed -> core registry -> managed callback -> core result path in the external E2E suite

Validation

• dotnet build -c Release --no-restore (0 warnings)
• targeted E2E: 1/1 passed
• full external E2E: 234/234 passed