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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." value. Using this value would be a bug. The value will be unused, or overwritten before being used."
</p> </p>
<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 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. implementation feature, not a language semantic, so it is not guaranteed to be observable to code.
</p> </p>