fix: avoid double free in external typed array creation

[GrapeBaBa]

Summary

• free the duplicated element buffer only for N-API errors returned before finalizer registration
• leave potentially transferred buffers exclusively owned by the external ArrayBuffer finalizer
• preserve original errors and keep existing external-memory accounting unchanged

Problem

TypedArray.fromExternal duplicates the input slice into allocator-owned native storage, then passes that allocation to napi_create_external_arraybuffer with a finalizer.

The existing catch-all frees the allocation for every external ArrayBuffer creation error. Node can return GenericFailure after it has already installed or invoked the finalizer, so the catch-all can free an allocation that the finalizer also owns.

Ownership boundary

• NoExternalBuffersAllowed, PendingException, and CannotRunJS: Node returns before finalizer registration, so free the duplicated buffer and propagate the original error
• other external ArrayBuffer errors: do not free because Node may already have installed or invoked the finalizer
• later external-memory accounting or TypedArray view failures: keep cleanup with the already-created ArrayBuffer finalizer

Node implementation references:

• https://github.com/nodejs/node/blob/v25.2.1/src/js_native_api_v8.h#L210-L221
• https://github.com/nodejs/node/blob/v25.2.1/src/node_api.cc#L946-L978
• https://github.com/nodejs/node/blob/v25.2.1/src/node_buffer.cc#L416-L441

Verification

• zig fmt --check src/js/typed_arrays.zig
• zig build test:zapi
• zig build
• pnpm test:js — 120/120
• pnpm lint:js
• git diff --check origin/main
• independent Zig ownership audit — PASS, 0 MUST / 0 SHOULD / 0 NIT

Coverage limitation

The existing Node addon tests cover the production success paths, but they cannot deterministically inject a post-registration GenericFailure or count finalizer callbacks. No production mock seam or compatibility path was added for those cases.