fix: isolate large array options per serializer

[lprnmns]

Problem

The large-array options are applied to module-level state, so building one customized serializer changes the behavior of later serializers that omit those options.

For example, after building with largeArrayMechanism: 'json-stringify' and largeArraySize: 1, a later default serializer uses JSON.stringify for a one-element integer array and throws on a BigInt. The default serializer normally handles that BigInt as documented.

Fix

Keep largeArrayMechanism and largeArraySize in the per-build context used during code generation. Explicit options remain unchanged, while each build starts with the documented defaults.

Tests

• node --test --test-name-pattern='large-array options do not leak between serializers' test/array.test.js - passed; the same regression failed before the fix
• npm test - passed; 502 unit tests, 100% reported coverage, 13 TypeScript assertions
• npm run lint - passed
• npm run benchmark - passed
• node --check index.js && node --check test/array.test.js - passed
• git diff --check - passed

Compatibility

An individual serializer's explicit large-array settings are unchanged. Serializers built without options no longer inherit settings from an earlier build. No package version or lockfile was changed.

Related issue

Independent reproduction; no current issue or open PR matching per-build large-array option isolation was found.