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>

scripts/benchmark/compare.sh

@@ -120,11 +120,11 @@ for key in all_keys:
     b_val = b_idx.get(key)
     a_val = a_idx.get(key)
 
-    b_str = f"{b_val:.1f}" if b_val else "—"
-    a_str = f"{a_val:.1f}" if a_val else "—"
+    b_str = f"{b_val:.1f}" if b_val is not None else "—"
+    a_str = f"{a_val:.1f}" if a_val is not None else "—"
 
-    if b_val and a_val:
-        delta_pct = (a_val - b_val) / b_val * 100
+    if b_val is not None and a_val is not None:
+        delta_pct = (a_val - b_val) / b_val * 100 if b_val != 0 else (float('inf') if a_val > 0 else 0)
         if delta_pct > 0:
             d_str = f"\033[32m+{delta_pct:.1f}%\033[0m"
         elif delta_pct < 0: