feat(libc): print the text that registers point at in Windows crash reports

Unreal titles end in their own libc fatal path with the UTF-16
LowLevelFatalError message in a register, and the crash report shows
only the first 64 bytes of it as hex words, so the message is lost
(#4238: Demon Slayer PPSA03490, Stray PPSA02100).

For every general register that points at readable memory starting
with at least four printable ASCII characters, as UTF-16LE or as single
bytes, the report now prints the text up to a NUL, the first other
character, 512 characters or the end of readable memory (each page is
checked with VirtualQuery before it is read).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/user/USAGE.md

@@ -206,6 +206,8 @@ Limits are listed in the [libc](../dev/debt/libc.md) and [relinker](../dev/debt/
 
 ### Windows crash diagnostics
 
+When a register points at readable memory that starts with at least four printable ASCII characters, either as UTF-16LE or as single bytes, the crash report prints that text after the register memory dump, as `[r13] u"..."` for UTF-16LE or `[rdi] "..."` for single bytes. The text ends at a NUL, at the first character that is not printable ASCII, a tab, a line feed or a carriage return, after 512 characters or at the end of readable memory. Tabs, line breaks, quotes and backslashes are escaped, and `...` after the closing quote means that no NUL ended the text within those limits.
+
 Set `APS5_CRASH_INDIRECT_MEMORY=1` before launching the executable to include up to 64 bytes from each pointer found in the register memory dump. The report labels each block with its register, pointer offset and target address. Inaccessible, incomplete and executable targets are skipped. This additional dump is disabled when the variable is unset or empty.
 
 ### GPU submission rings and stall dumps