Minimize diffs and improve backward compatibility
- chat-parser.h: Restore to match master (no changes)
- chat-parser.cpp: Keep COMMON_CHAT_FORMAT_PEG_* checks for backward compat
- Restore original function and switch statement order
- Add format-specific mapper selection for new parsers
- Revert LOG_INF back to LOG_DBG
- chat.cpp: Restore ifind_string to original location
- Reinstate PEG_SIMPLE/PEG_NATIVE/PEG_CONSTRUCTED format names
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
