Add toJSON() to Struct and document architectural trade-offs

[Copilot]

Architectural review of property descriptors vs proxies vs instance descriptors, with a concrete fix for the main ergonomic gap.

Analysis (ARCHITECTURE.md)

Prototype property descriptors remain the right call. Proxies would fix spread/JSON ergonomics but cost 5–10× on property access, break TypeScript inference, and don't compose with class/instanceof. Instance descriptors would fix spread but add per-instance GC pressure—bad for large struct arrays.

toJSON() on Struct

The biggest practical gap: JSON.stringify(struct) returned {} because it only sees own enumerable properties. toJSON() iterates inherited enumerable fields into a plain object. Nested substructs serialize recursively via the standard toJSON protocol.

const rect = new Rect(buf)
JSON.stringify(rect)
// {"origin":{"x":1,"y":2},"size":{"x":3,"y":4}}

// Also works as a spread workaround
const copy = { ...rect.toJSON() }

• core.ts — toJSON() method on Struct
• mod_test.ts — 3 tests: primitive fields, nested substructs, empty struct
• README.md — Updated gotcha Add toJSON, structToObject, pad, and enumField to structview #4 (JSON.stringify now works; spread still needs s.toJSON())
• CHANGELOG.md — 0.17.0 entry