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/common.bats

@@ -52,16 +52,18 @@ setup() {
 # ── data_dir ─────────────────────────────────────────────
 
 @test "data_dir: creates directory and returns path" {
+    local subdir="test-bats-$$-$RANDOM"
     local dir
-    dir="$(data_dir "test-tmp-$$")"
+    dir="$(data_dir "$subdir")"
     [ -d "$dir" ]
-    rmdir "$dir"
+    rmdir "$dir" 2>/dev/null || true
 }
 
-@test "data_dir: default returns data/ under project root" {
+@test "data_dir: returns path under PROJECT_ROOT/data" {
     local dir
-    dir="$(data_dir ".")"
-    [[ "$dir" == "$PROJECT_ROOT/data/." ]]
+    dir="$(data_dir "testdir")"
+    [[ "$dir" == "$PROJECT_ROOT/data/testdir" ]]
+    rmdir "$dir" 2>/dev/null || true
 }
 
 # ── logging functions produce output ─────────────────────
@@ -105,7 +107,7 @@ setup() {
     unset RED GREEN YELLOW BLUE CYAN BOLD DIM RESET
     source_lib common.sh
     # When stdout is not a tty (as in bats), colors should be empty
-    [ -z "$RED" ] || [ "$RED" = "" ]
+    [ -z "$RED" ]
 }
 
 # ── require_root ─────────────────────────────────────────