SP int: fixes from review by Claude

[SparkiDev]

Description

1. sp_cond_swap_ct_ex (line ~5524) — XOR typo: b->sign ^= b->sign always
   zeroed the sign. Fixed to b->sign ^= t->sign to correctly swap signs.
2. sp_mod_d (line ~7271) — Negative modulo correction was applied even
   when the remainder was 0. Added (*r != 0) guard to avoid producing d
   instead of 0.
3. sp_lshb (line ~8444) — Left-shift size check was off. Refactored to
   correctly distinguish between pure-digit shifts and bit-within-digit
   shifts when checking if the result fits, using separate overflow checks
   for each case.
4. _sp_mulmod_tmp (line ~12160) — Zero inputs caused an allocation of
   size 0, which is problematic. Added an early path: if either operand is
   zero, set result to zero and skip the allocation/multiply entirely.
5. sp_mod_2d — copy path (line ~14762) — XMEMCPY copied digits *
   SP_WORD_SIZEOF bytes but a may have fewer than digits used digits. Fixed
   to copy min(a->used, digits) digits to avoid reading uninitialized
   memory.
6. sp_mod_2d — negation loop (line ~14782) — Negation loop iterated
   over r->used, which could exceed digits. Fixed to loop over min(r->used,
   digits).
7. _sp_sqrmod (line ~17314) — Same zero-input issue as _sp_mulmod_tmp.
   Added early zero path to skip the allocation/squaring when input is
   zero.
8. sp_lcm (line ~19838) — Typo in sign check: b->sign >= MP_NEG
   (comparing against a value that is 1, so >= 1 would also match MP_ZPOS)
   changed to b->sign == MP_NEG.

Testing

Standard

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• gcc-arm-cortex-m4- gcc-arm-cortex-m4-baremetal- gcc-arm-cortex-m4-min-ecc- gcc-arm-cortex-m4-tls12