perf(dsl): skip placeholder pattern in materializeClassInstance

[spiral-ladder]

Summary

Optimize DSL class-return materialization by removing the internal placeholder instance path.

Previously, returning a DSL class from a function/method created a JS instance with an internal external-marker argument, allocated and wrapped a temporary placeholder native object, removed that wrap, destroyed the placeholder, then wrapped the real native object.

This PR changes materialization to call the class constructor directly in an internal materialization mode, skip normal init, and wrap the real native object after napi_new_instance returns.

Details

• Avoids placeholder native allocation/wrap/remove/destroy during class returns
• Removes now-unused placeholder finalizer hint handling
• Preserves subclass constructors for same-class returns
• Rejects cross-class or non-Zapi constructors during materialization
• Rejects subclass constructors that return a replacement object
• Ensures returned native resources are deinitialized if materialization fails
• Keeps static class field support from current main

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Analysis from @spiral-ladder

Per-call cost of every factory method that returns a DSL class (PublicKey.fromBytes, Signature.aggregate, SecretKey.sign, etc.) was:

1 napi_external alloc (V8 young-gen)
1 napi_new_instance -> constructor allocs placeholder native + napi_wrap
1 removeWrapChecked -> tears down placeholder finalizer entry
1 destroyInternalPlaceholder
1 native alloc for the real object
1 napi_wrap + typeTagObject

That's 2 native allocs + 1 free, 2 napi_wraps + 1 unwrap, 2 typeTagObject writes, and an extra V8 young-gen object per call. In lodestar's attestation/signature hot path this measurably increases Scavenge time.

Replace with a threadlocal marker: materializeClassInstance sets materialize_target before calling napi_new_instance(argc=0, args=null). The generated constructor early-returns when isMaterializing(T) is true, skipping the placeholder alloc/wrap entirely. materialize then wraps the real object directly.

After patch: 1 native alloc + 1 napi_wrap + 1 typeTagObject per call.

Bench (lodestar-z bench/napi/materialize.bench.mjs, 200k iters):
PublicKey.fromBytes 3.28% -> 2.10% Scavenge (-1.18 pp, -69% count)
Signature.fromBytes 0.78% -> 0.47% Scavenge (-38% count)

[spiral-ladder]

Analysis from @spiral-ladder

Analysis from claude, to be precise ;)