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langref: undefined _is_ materialized in all safe modes (#24107)

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* trailing whitespace

* langref: undefined _is_ materialized in all safe modes

I am also not super happy about the clause that immediately follows. I
_believe_ what we want to say here is that, simultaneously:

* undefined is guaranteed to be matrerialized in in all safe modes.
  A Zig implementation that elides `ptr.* = undefined` in ReleaseSafe
  mode would be a non-conforming implementation.
* A Zig program that relies on undefined being materialized is buggy.

But I don't think it's the time to engage this level of language-lawering!
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@ -777,7 +777,7 @@
value. Using this value would be a bug. The value will be unused, or overwritten before being used."
</p>
<p>
In {#link|Debug#} mode, Zig writes {#syntax#}0xaa{#endsyntax#} bytes to undefined memory. This is to catch
In {#link|Debug#} and {#link|ReleaseSafe#} mode, Zig writes {#syntax#}0xaa{#endsyntax#} bytes to undefined memory. This is to catch
bugs early, and to help detect use of undefined memory in a debugger. However, this behavior is only an
implementation feature, not a language semantic, so it is not guaranteed to be observable to code.
</p>
@ -2295,7 +2295,7 @@ or
{#code|test_aligned_struct_fields.zig#}
<p>
Equating packed structs results in a comparison of the backing integer,
Equating packed structs results in a comparison of the backing integer,
and only works for the {#syntax#}=={#endsyntax#} and {#syntax#}!={#endsyntax#} {#link|Operators#}.
</p>
{#code|test_packed_struct_equality.zig#}