fix+test: improve test suite, fix 2 bugs found by tests

Bugs fixed in production code:
- compare.sh: Python truthiness on 0.0 — `if b_val` was False for 0.0 t/s,
  displaying it as a dash instead of "0.0". Fixed with `is not None` checks.
- compare.sh: ZeroDivisionError when computing delta % with 0.0 baseline.

Test improvements (review findings):
- detect.bats: kernel param tests now use real detect_kernel_param logic
  pattern (not a separate reimplementation). Added non-GiB-aligned RAM test,
  device ID without 0x prefix, empty firmware version, llama-bench detection,
  detect_total_physical_ram_kb tests.
- benchmark_compare.bats: assert delta percentages (+20.0%, -25.0%, 0.0%),
  test 0.0 t/s edge case, test per-directory error messages, test config
  change detection with specific field assertions.
- log_metrics.bats: add assert_success, --help test, timestamp format
  validation. Remove unused mock sysfs setup.
- common.bats: fix data_dir test, remove redundant assertion, add cleanup.
- test_helper.sh: remove unused FIXTURES_DIR.
- Remove empty tests/fixtures/ directory.

94 tests, all passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_helper.sh

@@ -9,7 +9,6 @@ load "$BATS_SUPPORT/load.bash"
 load "$BATS_ASSERT/load.bash"
 
 PROJECT_ROOT="$(cd "$(dirname "${BATS_TEST_FILENAME}")/.." && pwd)"
-FIXTURES_DIR="$PROJECT_ROOT/tests/fixtures"
 
 # Create a temporary mock sysfs tree for GPU detection tests
 setup_mock_sysfs() {