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fix(deque): handle overlapping self-blit in blit_to

When blit_to is called with src == dst and overlapping regions where
src_offset < dst_offset, the source elements were being overwritten
before being read, causing incorrect results.

Example: d = [1,2,3,4,5], d.blit_to(d, len=3, src_offset=1, dst_offset=2)
Before: [1, 2, 2, 2, 2] (wrong - source overwritten during copy)
After:  [1, 2, 2, 3, 4] (correct - copied in reverse order)

The fix detects this case using physical_equal and copies in reverse
order when needed, similar to how memmove handles overlapping regions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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