Improve alignment promise for empty AlignedData

[FreezyLemon]

#74 uncovered that empty data/len==0 returned a dangling pointer that was not following the same alignment as real allocations. This required some doc changes and is just not nice in general.

Use layout.dangling_ptr instead of the less specific NonNull::dangling_ptr to follow our "usual" alignment rules exactly. This requires an MSRV bump to 1.95.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.40%. Comparing base (59bc0dc) to head (d84c738).

Files with missing lines	Patch %	Lines
src/plane/aligned.rs	93.54%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #76   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files           7        7           
  Lines         668      676    +8     
=======================================
+ Hits          664      672    +8     
  Misses          4        4           

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