Interrupt-safe ring-buffer snapshot

When an interrupt handler is the sole writer of a fixed ring buffer, foreground code can obtain a coherent view by saving and masking the relevant interrupt, copying the count and valid entries, and restoring the earlier mask. Perform expensive calculations on the copy after leaving the critical section. Exclusion prevents the writer from changing the logical version during the copy; individually atomic loads alone do not give this guarantee. Correctness also requires compiler ordering at the boundaries, no unmasked competing writer, and a short enough mask interval that the peripheral does not lose events. Restoring the previous mask avoids accidentally enabling an interrupt disabled by an outer caller.