fix: root registered-callback delegates and fix metadata handle ownership

[tinysec]

Round-19 audit: delegate lifetime + metadata handle ownership

1:1 comparison against the official C++/Python binaryninja-api found two classes of lifetime bugs where the GC could reclaim a delegate (or strand a native ref) while the core still held the function pointer / handle.

1. Callback delegate lifetime (CallbackOnCollectedDelegate → AccessViolation)

Every site handing a function pointer to a core API that stores it long-term built the adapter delegate inline, leaving no rooted reference. Once the GC collected the temporary, the native pointer dangled and the next callback crashed. Fixed across:

• 5 Custom* ToNative structs (CustomBinaryView, CustomBinaryViewType, CustomTransform, FileAccessor, LogListener): per-instance thunk fields.
• BNLogListener.FromNative: copy-paste fix (close/getLogLevel had pointed at log).
• 11 Custom*Command instance Register*() + 14 static RegisterPluginCommand*: thunk fields / shared UnsafeUtils.PinCallback.
• Core.RegisterBinaryViewEvent, BinaryViewType.RegisterPlatformRecognizer, FlowGraph.StartLayout (async completion): PinCallback process-lifetime root.
• Activity.Create/CreateWithEligibility: per-instance delegate fields.
• 22 UnsafeUtils.WrapProgressDelegate call sites: local + GC.KeepAlive across the synchronous native call.

New UnsafeUtils.PinCallback<T> centralizes the process-lifetime rooting so every registration site routes through one obviously-correct helper.

2. Metadata native-handle ownership

• Metadata.ToArray + MetadataValueStore.FromNative: MustTakeHandle (no addref on a transferred-owned ref; the core frees only the container).
• 12 scalar/array Metadata.SetValue overloads: wrap the BNCreate* handle in an owning Metadata (using) and pass a borrowed handle; the core addrefs internally so the value survives the wrapper's disposal. (Previously leaked one BNMetadata ref per call.)

Validation

• Build: 0 warnings / 0 errors.
• e2e (outside repo, per project constraint): 1470 passed / 0 failed / 3 skipped (pre-existing absent-export skip). +2 modules: CallbackLifetime (the logListenerSurvivesGc probe would crash the process pre-fix; now passes on all 3 samples) and MetadataSetValueOwnership (all 8 probes pass, incl. survivesGc).