Three unit tests for the BufferPool zero-fill fix:
1. recycled_buffer_is_fully_zeroed_after_shrink: writes a 'secret',
truncates to 8 bytes, drops to recycle, verifies the entire
recycled buffer is zero. Catches the original v.fill(0)+set_len
regression where the tail bytes were uninitialized.
2. recycled_buffer_zeroed_at_known_nonzero_size: writes 0xAB across
the full buffer, truncates, recycles, verifies all bytes are 0.
Catches the same regression from a different angle.
3. fresh_buffer_is_zeroed: trivial check that a brand-new buffer
from a fresh pool is already zeroed (sanity check).
The netstack crate is redox-target-only (cannot run on host due to
redox_syscall/redox-scheme dependencies), but these tests serve as
documentation of the expected behavior and can run in the canonical
build-redbear.sh target test pass.
Red Bear OS
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