Capturing a shader threw "null or misaligned address" when the flat
SRT walk read through a pointer loaded from memory that is null or
unmapped, even when the program skips that s_load. Such a read now
yields zero and a record with its pc and address, as does a slot whose
address needs it. The registered artifact checks one flag word per
guarded slot at each ReadConst; the capture appends the flags and the
records to the flattened SRT, and a set flag raises an Unmapped BDA
fault through the fault buffer and stops the invocation as a BDA read
does. Nothing per capture enters a key: poisoned and clean captures
share one variant and specialization. Descriptors, branch conditions,
uniform fills, table bases and user-data pointers that need such a
value still throw, and poisoned results stay out of the dispatch and
draw caches. The shader disk cache format becomes 21.
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