Fix D+/D-/DNEGATE: use ucell_t instead of unsigned long (FABRIC-3.md §XII.4)

double_word_d_plus(), double_word_d_minus(), and double_word_dnegate()
(double_words.c) all cast through plain `unsigned long` for their carry/
borrow-detection arithmetic. On this aarch64 bare-metal cross-compile
target, unsigned long is 32-bit (confirmed: sizeof(unsigned long)==4) --
amd64 and riscv64 both happen to have a 64-bit long, so the identical code
only broke on aarch64. The low-cell arithmetic silently truncated to 32
bits, then widened back to cell_t via ordinary (non-sign-extending)
conversion, producing a wrong result whenever the true 64-bit result was
negative -- D. then correctly, faithfully reported DOUBLE-OVERFLOW on the
resulting malformed double.

vm.h already defines ucell_t for exactly this: same conditional as cell_t,
guaranteed width-matched on every target. print_number_formatted()
(format_words.c) already used it correctly; these three words didn't.
Switched all three to ucell_t -- a one-word-class fix, no logic change.

Verified: rebuilt and booted all three architectures clean. T19 (D+) on
aarch64 now correctly prints -2, matching amd64/riscv64; T20 (DNEGATE)
unaffected everywhere. Additional manual cases beyond the original
exerciser, run live on aarch64 to specifically exercise the
>32-bit-magnitude path the old bug depended on: D- (-5-3=-8), DNEGATE on
2^33 (8589934592 -> -8589934592), D+ crossing the same boundary
(3+8589934592=8589934595) -- all correct.

d_compare() (backing DMAX/DMIN/D</D=) has the identical latent pattern but
is out of scope for this fix (not named in the request, never exercised by
the campaign) -- left open, flagged in FABRIC-3.md. M*'s separate,
universal-across-all-three-architectures DOUBLE-OVERFLOW bug is also
untouched -- unrelated defect, not part of this fix.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EXieurDfDSsDFdnSyusuWo

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