feat(bytesbuf): add BytesBuf::split_off_remaining

[sandersaares]

Summary

Add split_off_remaining() and split_off_remaining_checked() methods to BytesBuf that detach a specified number of bytes of remaining (unfilled) capacity from the end of the buffer, returning a new empty BytesBuf that owns that capacity. The original buffer's filled data is unaffected.

This is a utility function that can be beneficial in test code and in specialized situations where cutting buffers to specific capacities is useful — for example, to pre-partition capacity across multiple buffers from a single reservation.

API

// Panics if count > remaining_capacity().
pub fn split_off_remaining(&mut self, count: usize) -> BytesBuf;

// Returns None if count > remaining_capacity().
pub fn split_off_remaining_checked(&mut self, count: usize) -> Option<BytesBuf>;

Follows the established crate pattern of panicking + checked variant pairs (like consume/consume_checked and begin_vectored_write/begin_vectored_write_checked). The naming follows the split_off convention from the standard library (Vec::split_off, String::split_off).

Changes

• span_builder.rs: Added internal SpanBuilder::split_off_available() helper that splits off unfilled capacity from the end of a span builder into a new SpanBuilder sharing the same memory block.
• buf.rs: Added split_off_remaining() and split_off_remaining_checked() on BytesBuf. The implementation drains complete empty span builders from the end and, if needed, splits a partially-used builder's available capacity.
• Unit tests for all new methods covering: basic usage, multi-block buffers, writing to split-off buffers, zero-count edge case, full-capacity split, panic on overflow, and checked-returns-None.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (496da35) to head (7c241c5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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##             main     #286   +/-   ##
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  Coverage   100.0%   100.0%           
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  Files         141      141           
  Lines        8664     8715   +51     
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+ Hits         8664     8715   +51     

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[ZacWalk]

Looks good to me. Might be good to add a test for split_off_remaining on an empty buffer with no capacity (Just a suggestion though)