Make unsafe borrow checking O(1) via active-borrow tracking

[Copilot]

Borrow mutation checks in unsafe code were doing dynamic scans across locals/fields/slots, making enforcement cost scale with context size. This change moves borrow checking to constant-time lookups while preserving current pointer-safety behavior.

• Core change: replace scan-based checks with indexed borrow state

  • Added active-borrow index in ExecutionContext keyed by container identity + element index.
  • AssignmentHandler.ensureNoActiveBorrow(...) now queries ctx.hasActiveBorrow(container, index) instead of traversing runtime maps/lists.

• Tracking lifecycle wired into all relevant mutation paths

  • Added tracked replacement flow so borrow counts stay accurate when values move or are overwritten.
  • Integrated tracking updates into:
    • local variable set/remove/scope pop,
    • slot writes,
    • object field writes (via new setObjectField path),
    • indexed container writes where replaced values may contain nested pointers.

• Assignment path cleanup for consistency

  • Updated field and scoped-variable assignment sites to use context-managed write APIs, ensuring borrow tracking and state mutation remain coupled.

private void ensureNoActiveBorrow(Object container, long index, ExecutionContext ctx) {
    if (!isBorrowCheckerActive(ctx)) return;
    if (ctx.hasActiveBorrow(container, index)) {
        throwBorrowMutationViolation(index);
    }
}