[pointer] Require same-size proof in transmute helper

[joshlf]

Close the latent soundness hole in impl_for_transmute_from! by making its missing size premise explicit.

• Add an internal unsafe SameSizeForTransmute<R> proof trait whose contract is a property of the two types alone: Self and R have exactly the same set of possible referent byte lengths. For two Sized types, this reduces to ordinary size_of equality.
• Make every SameSizeForTransmute impl prove that contract locally from the relevant representation/size guarantee. These proofs do not depend on impl_for_transmute_from!, unsafe_impl_for_transparent_wrapper!, or another zerocopy unsafe assertion.
• Require SameSizeForTransmute before reciprocal TransmuteFrom<_, Safe, Safe> bounds are used. For an arbitrary possible referent size, the marker makes both TransmuteFrom contracts applicable; the reciprocal bounds then establish equality of the two Safe state sets at that size.
• Spell out separately how that result transfers each supported trait: FromZeros, FromBytes, IntoBytes, and TryFromBytes.
• Provide witnesses for every existing representation family used by impl_for_transmute_from!: Wrapping<T>, ManuallyDrop<T>, Cell<T>, UnsafeCell<T>, the integer/bool atomic types, and AtomicPtr<T>.
• Remove the [pointer] Require size equality in impl_for_transmute_from! #3691 FIXME now that its missing premise is represented directly.

This deliberately does not change SizeEq; that trait still carries no safety guarantee of its own.

Fixes #3691.

Authored by an AI agent acting on Josh Liebow-Feeser's behalf.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.90%. Comparing base (1f5c93a) to head (b24150d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3694   +/-   ##
=======================================
  Coverage   91.90%   91.90%           
=======================================
  Files          20       20           
  Lines        6175     6175           
=======================================
  Hits         5675     5675           
  Misses        500      500           

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