feat(searching): add exponential_search2 with self-tests

[hobostay]

#- What: Add searching/exponential_search2.c as an alternative exponential search implementation with Doxygen header and assert-based self-tests.

• Why: Complement existing exponential_search.c with clear range expansion and bounded binary search; verifies edge cases.
• Tests: Includes assert tests for present/absent values, boundaries, and empty input.
• Build: Picked up by searching/CMakeLists.txt via glob; C11 compliant; no external deps.

[hobostay]

• Complexity: Time O(log n) via exponential range expansion + binary search; Space O(1) .
• Edge cases: empty array, out-of-range targets, boundaries covered by asserts.