[layout] Restrict CastFrom's CastExact impl

[enthropy7]

CastExact promises that a cast preserves the complete referent byte range. However, CastFrom only preserves the address and the size before trailing padding. This is incorrect for dynamically sized types with explicit alignment:

#[repr(C, align(8))]
struct Aligned<T: ?Sized> {
    trailing: T,
}

An Aligned<[u8]> containing one byte has a total size of eight bytes because of trailing padding. Projecting it to [u8] produces a one-byte referent. This is a valid shrinking Cast, but not a CastExact.тThis case came up while checking complete byte ranges in aligned zero-copy video buffers, instead of checking only addresses and logical payload lengths.

This PR restricts the CastExact implementation to Src: IntoBytes. Such sources cannot contain padding, so the complete byte range is preserved. The general Cast implementation remains available for projections that may shrink.

No current safe public path to UB was found because the existing transmute paths already require Src: IntoBytes. The change is still needed because the unsafe CastExact implementation must be correct for every type for which it is implemented. Otherwise, future internal code could rely on a false size-preservation guarantee.

The regression test demonstrates the eight-to-one-byte projection and verifies that it implements Cast, but not CastExact.

Tested with the repository's MSRV, stable, and nightly configurations, including Clippy, formatting, and all 136 library tests.

Related to #2910 and #3199.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.86%. Comparing base (82eed02) to head (efc46c3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3549   +/-   ##
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  Coverage   91.85%   91.86%           
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  Files          20       20           
  Lines        6093     6100    +7     
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+ Hits         5597     5604    +7     
  Misses        496      496           

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