Vector int64 +, -, *, % still round through double above 2^53 (div was fixed in #403)

[singaraiona]

Summary

#403 made integer div exact above 2^53, but the sibling ops in the same binary_range I64 arm still compute through double and round:

(+ [9007199254740993 -9007199254740993] [1 1])  ;; [9007199254740993 -9007199254740991] — want ...994 / ...992
(- [9007199254740993] [1])                       ;; 9007199254740991 — want ...992
(* [9007199254740993] [1])                       ;; 9007199254740992 — want ...993
(% [9007199254740993 9007199254740995] [3 3])    ;; [2 0] — want [0 2]
(div [9007199254740993] [3])                     ;; exact since #403

Identical in builds before and after #403 — not a regression, just the remaining chapter of the same story.

Notes

• The fix(arith): keep integer div exact and UB-free #403 pattern transfers directly: an i64_span_dbl_exact (±2^53) gate keeping the vectorized double loop for the common case, an exact int64 kernel otherwise. For +/-/* the exact kernel needs overflow handling consistent with the atoms' existing convention (check what scalar + does at INT64_MAX today and match it).
• When picking this up, also fold in the gate-scan cost lesson from fix(arith): keep integer div exact and UB-free #403's review: don't scan broadcast scalars per element, scan vector operands through typed pointers, fuse the operand passes.

[singaraiona]

Sharper scope from the follow-up review: it's not just arithmetic — I64 comparisons round through double above 2^53 too:

(+ 9007199254740993 0)                  ;; 9007199254740992 — wrong
(!= 9007199254740992 9007199254740993)  ;; false — compares EQUAL

Both stem from the same (int64_t)LV_READ(i) double round-trip in binary_range's I64 and BOOL arms. The comparison case is the more dangerous one: it silently invalidates any test or query that checks large-int values with ==/!=, which is why the div-gate test fixtures had to be built from cycled literals rather than arithmetic. After #403 + 9e5f0a1, div is the only exact binary op above 2^53.