Vitte Ownership and Borrow (Core Freeze 0.1)

Stable borrow semantics checked by the mandatory borrowck phase:

• move invalidates previous owner uses (E_BORROW_USE_AFTER_MOVE).
• borrowed value cannot outlive owner (E_BORROW_OUTLIVES_VALUE).
• mutable and shared borrows cannot alias simultaneously (E_BORROW_MUT_AND_SHARED).
• borrowck is critical: skipped borrowck fails the build gates.

Advanced borrow features (NLL, reborrow graph precision, partial moves) remain experimental.