fix: safety hardening for protocol implementation

[w4bremer]

Summary

• Harden ObjectLink protocol against malformed input, crashes, and unsafe patterns
• Fix instance isolation bugs (shared class-level state, registry singletons)
• Fix broken example code (wrong imports, setattr, arg unpacking)
• Add 30 safety tests covering all fixes

Changes

Critical fixes

• Fix broken setattr call in examples/server.py
• Fix shared class-level node/queue in examples/server.py RemoteEndpoint
• Fix broken import paths (olink.remotenode → olink.remote) in examples
• Fix wrong **args → *args unpacking in examples/server.py

Safety hardening (wire protocol unchanged)

• Validate message length before unpacking in Protocol.handle_message (silent drop, >= for forward-compat)
• Replace unsafe getattr/setattr with explicit dispatch tables in examples
• Fix sender task deadlock on disconnect (cancel task before join)
• Prevent phantom registry entries (_get_entry for reads, _entry for writes)
• Fix KeyError on double-unlink and unregister of unknown sink
• Add configurable message size limit (1MB default)
• Narrow exception handling from except Exception to except (json.JSONDecodeError, ValueError)
• Forward node logger to Protocol for validation error visibility

Instance isolation fixes

• Ensure independent state for ClientNode, ClientRegistry, RemoteRegistry, MockSink, MockSource, RemoteEndpoint instances
• Fix self parameter in interface methods
• Filter inf values from JSON serialization

Cosmetic

• Fix ProptocolType typo, enum tuple syntax, missing self in ILogger.log

Test plan

• All 31 original tests pass
• 30 new safety tests pass (malformed messages, registry edge cases, phantom entries, size limits)
• Zero wire protocol changes — other language implementations unaffected
• Manual test with starlette/uvicorn (examples require starlette installed)