fix(native): napi-rs null visibility crashes insertNodes

[carlos-alm]

Bug

The native (Rust) engine crashes during insertNodes when a definition or child node has visibility: null:

Error: Failed to convert JavaScript value `Null` into rust type `String`
  on InsertNodesDefinition.visibility on InsertNodesBatch.definitions

Root cause

In insert-nodes.ts, the marshaling code coerces undefined to null:

visibility: def.visibility ?? null,

napi-rs v3 #[napi(object)] for Option<String> handles undefined → None correctly, but when it sees null, it attempts to convert it as String (not None), which fails.

Parameters and some children created by extractors don't set a visibility field (it's undefined in JS). The ?? null coercion turns this into an explicit null, triggering the napi conversion error.

Impact

• Native engine cannot build any graph — complete build failure
• v3.5.0 benchmarks have "native": null for all benchmark types
• Affects all codebases (parameters are universal)

Fix

Remove ?? null coercion for Option<String> fields in the native marshaling path. Let undefined flow through naturally so napi-rs converts it to None.