RFC(runtime): shared objects across JS workers and Home-native threads

Problem

Workers can only share raw SharedArrayBuffer bytes; every object crossing a thread is copied. The engine is Bun's prebuilt WebKit, which has no SharedStruct, and the optional zig-js engine lives outside this repo. There is also no way to load Home modules from JS. "Threads with shared objects" needs an engine-level answer. (New direction.)

Evidence (origin/main @ d4b219507)

• build.zig:95-120
• build.zig:315
• build.zig:409

Scope

• Evaluate: (a) generate JS accessor classes from Home struct layouts, backed by a SAB and using Home's locks/atomics; (b) implement the TC39 shared-structs proposal (shared structs, shared arrays, Atomics.Mutex/Condition) in zig-js or a JSC fork; (c) Home modules loadable from JS with Sync types exposed as shared objects.
• Define the memory model (JS SAB model vs. Home orderings) and the GC interaction for shared objects.

Acceptance criteria

• RFC merged in docs/docs/DECISIONS.md.
• Prototype: two JS workers and one Home-native thread mutate one struct under a mutex with no lost updates.

Links

Depends on: #802, #807, #810 Related: #806, #786 Part of #814.