CHAR_MIN and CHAR_MAX ignore the target's char signedness

Found by the cross-target differential, 2026-09-27 (crtl_libc_oracle row CHAR_MIN=-128 CHAR_MAX=127 char-signed=0 on aarch64/arm32/riscv32).

Resolution (2026-09-28)

The preprocessor predefines __CHAR_UNSIGNED__ where CPlainCharSigned is false, as gcc does; lib/crtl/include/limits.h keys CHAR_MIN/CHAR_MAX on it. UCHAR_MAX and USHRT_MAX (limits.h) and UINT8_MAX and UINT16_MAX (stdint.h) are plain int constants. Fixture test/c_limits_follow_the_targets_char.c (17 rows stated as rules): v445 5 rows wrong on the unsigned-char targets, and 4 on every target with the old headers; this build none.