Makefile: bench-fork-score CLI invocation v8 → substrate

Caught while sweeping for remaining "arborist v8" CLI invocations
after bae5caf renamed the subcommand. Makefile target
``bench-fork-score`` (line 430) still called
``$(ARBORIST) v8 score`` directly — would have failed loudly on
next ``make bench-fork-score`` run with an argparse error
("invalid choice: 'v8'").

Same kind of subtle breakage I caught in fork_score.py earlier
today (85be5eb): a refactor's text-search misses a callsite that
isn't picked up by Python imports. Lesson: after renaming a CLI
subcommand, grep ``$(ARBORIST) <oldname>`` AND ``arborist <oldname>``
in Makefile, scripts/, and bench/ — argparse errors don't surface
until someone runs the target.

Hygiene
=======
- ``grep -rn "arborist v8\\b" Makefile bench scripts docs`` → only
  the historical-note comment in cli.py:5131 remains.
- ``make help | grep substrate`` → target descriptions accurate.
- ``.venv/bin/arborist substrate score --help`` still 0.

Makefile

@@ -427,7 +427,7 @@ bench-fork-score: bootstrap ## #000012 Phase 1b — score current bench output v
 	@mkdir -p bench/results
 	PYTHONUNBUFFERED=1 $(PY) -m bench.batteries.runner --all \
 	    --out $(FORK_CHILD)
-	$(ARBORIST) v8 score \
+	$(ARBORIST) substrate score \
 	    --parent $(FORK_PARENT) \
 	    --child $(FORK_CHILD) \
 	    --out $(FORK_REPORT)