feat(types): Send/Sync auto traits and thread-aware borrow checking

Problem

Data-race freedom is not enforced anywhere, yet ROADMAP-WEB-COMPETITIVE.md marks "No data races (Send/Sync) – Done ✅". The built-in traits are Copy/Clone/Display/Debug/Default/Eq/Ord/Hash only; Send/Sync appear only in packages/safety/src/unsafe_blocks.zig, which isn't built. borrow_check_pass.zig tracks moves and borrows but knows nothing about threads or captures that cross them. Threads with shared objects are only safe with this. (New direction.)

Evidence (origin/main @ d4b219507)

• docs/docs/ROADMAP-WEB-COMPETITIVE.md:744
• packages/traits/src/trait_system.zig:170-220
• packages/safety/src/unsafe_blocks.zig:332-343
• packages/compiler/src/borrow_check_pass.zig

Scope

• Send / Sync auto traits derived structurally; raw pointers and non-atomic RC opt out.
• spawn requires Send captures; shared references across threads require Sync.
• Borrow checker: no &mut shared across threads; scoped threads may borrow.

Acceptance criteria

• Sending a non-Send value into spawn is a compile error with a dedicated code and test.
• Sharing &mut across threads is rejected; Arc<Mutex<T>> is accepted.

Links

Depends on: #805 Part of #814.