feat(lang): `spawn`, threads and a multi-core executor in the Home language

Problem

Home code has no way to run in parallel. The lexer has no spawn keyword. packages/threading/src/bindings/sync.home (983 lines) declares @extern fn thread_spawn/mutex_create/..., but nothing exports those symbols. The interpreter's WorkerPool.spawn runs closures synchronously, and awaiting a pending future quietly returns Void. Native codegen says "We don't yet have an executor" — block_on busy-polls. examples/wip/async_example.home already uses spawn / Mutex<T>. (New direction.)

Evidence (origin/main @ d4b219507)

• packages/lexer/src/token.zig:393-455
• packages/threading/src/bindings/sync.home
• packages/interpreter/src/interpreter.zig:7980-8000
• packages/interpreter/src/interpreter.zig:1790
• packages/codegen/src/native_codegen.zig:6122-6137
• packages/codegen/src/native_codegen.zig:6531-6570
• examples/wip/async_example.home

Scope

• Language: spawn (thread) and task spawning on a multi-core executor; JoinHandle, scoped threads.
• Export the sync.home extern symbols from the runtime (Mutex, CondVar, channels, Arc).
• Interpreter: real threads; awaiting a pending future blocks or errors, never returns Void.
• Native: a real executor instead of busy-poll block_on.

Acceptance criteria

• A parallel sum in .home over 8 threads is ≥4× faster than single-threaded on native and interpreter builds.
• examples/wip/async_example.home moves out of wip/ and runs.

Links

Depends on: #802, #804, #807 Part of #814.