feat: add named loggers, logger reset, and managed prefix registry

[privateip]

• Add get_logger(name) to return named child loggers under the ipsdk namespace, with ValueError guard against double-prefixing
• Add reset_logger(name) to strip handlers, reset level to NOTSET, and restore propagation on any stdlib logger by name
• Add register_logger_prefix(prefix) to extend the managed-logger registry beyond the hardcoded ipsdk/httpx prefixes
• Extend set_level() and initialize() with optional loggers parameter for one-shot per-call management without registry persistence
• Add OpenSpec change artifacts (proposal, design, specs, tasks) for the logging enhancements