Build with clang, and the core and store on macOS, again

Apple clang 21 (an M3) and clang 19 refused what gcc let pass (research
029):
- PURE_FIELD_REST was an enum of UINT32_MAX, past an int (C17): a macro.
- a store's publish copied an entry rebuild left unset when the record
  was not found (the copy was never used): copied when it was made.
- the log's image writer kept a count only Linux's sync_file_range uses.
- ",{" + (row == from) read as adding an int to a string: said plainly.
- the tests: mkdtemp, hidden by _POSIX_C_SOURCE on macOS, needs
  _DARWIN_C_SOURCE; a string split so that \x80 and a stay apart is
  bracketed; a count left over and a malloc measure only glibc has.
The core's and the store's tests pass on the Mac.

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