fix(algo): gate icebug behind ICEBUG_ENABLED, fail soft on download error - #50

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- Top-level CMakeLists adds option(ICEBUG_ENABLED ON) so icebug can be
  disabled explicitly (-DICEBUG_ENABLED=OFF) or as a fallback when the
  GitHub prebuilt download fails (offline CI, rate limit, etc.).
- download_icebug.cmake early-returns on -DICEBUG_ENABLED=OFF and, on a
  failed file(DOWNLOAD), emits a WARNING, flips ICEBUG_ENABLED to OFF
  (FORCE'd so re-configures don't re-fail), and returns with empty
  ICEBUG_* vars instead of fatal-erroring configure.
- Top-level CMakeLists gates find_package(Arrow), the icebug include
  dirs, lib linkage, and rpath behind ICEBUG_ENABLED. find_package(OpenMP)
  stays unconditional (other algos use <omp.h>).
- function/CMakeLists drops gds_page_rank.cpp + Arrow::arrow_shared when
  disabled; main/CMakeLists + algo_extension.cpp gate the GDS_PAGE_RANK
  registration and ARROW_DEFAULT_MEMORY_POOL setup behind #ifdef
  ICEBUG_ENABLED via target_compile_definitions.