feat: integrate memory safety infrastructure into compilation pipeline

- Add LinearTypeChecker integration to all compilation entry points
  - commands.rs, zyn_grammar.rs, typed_ast_json.rs
  - Can be bypassed with SKIP_LINEAR_CHECK=1 env var

- Create HIR-level borrow checking pass (borrow_check.rs)
  - HirBorrowChecker for traversing HIR and validating borrow rules
  - BorrowError enum for various violations (UseAfterMove, MutableBorrowConflict, etc.)
  - Integrated into compile_to_hir() via enable_borrow_check config option

- Add operands() method to HirInstruction for analysis passes

- Add run_linear_type_check() function to compiler lib.rs

Author: darmie