feat(int): add DigitSum builtin

[msollami]

Summary

• Adds DigitSum[n] — sum of the base-10 digits of |n|
• Adds DigitSum[n, b] — sum of the base-b digits of |n| (b ≥ 2)
• Sign discarded; DigitSum[0] → 0; bignum inputs supported via GMP

Changes

• src/int.{c,h} — builtin_digitsum() implementation (fast ulong path via mpz_tdiv_q_ui; slow bignum-base path via mpz_tdiv_qr); registered in int_init() with attributes Listable | NumericFunction | Protected
• src/sym_names.{h,c} — SYM_DigitSum interned symbol
• src/info.c — docstring
• tests/test_digit_sum.c — 33 tests: basic decimal, zero, negatives, two-arg base form, cross-check vs Total[IntegerDigits[n]], bignum inputs (10^30, 100!, 2^100), arity/type diagnostics, symbolic silence, Listable threading, attributes, docstring, 50-iteration stress loop
• tests/CMakeLists.txt — digit_sum_tests target
• docs/spec/builtins/arithmetic.md — DigitSum section
• docs/spec/changelog/2026-06-22.md — changelog entry

Testing

• make USE_ECM=0 — clean build, no warnings
• cd tests/build && make digit_sum_tests && ./digit_sum_tests — all 33 tests pass
• REPL: DigitSum[1234] → 10, DigitSum[255, 16] → 30, DigitSum[{1,99}] → {1, 18}

Out of scope

• No 3-arg form (Mathematica has no DigitSum[n, b, ...] variant)
• No symbolic simplification rules