yak(make-checkverify-format-gate-rewrites-files-instead-of-checking-lnje): make check/verify format gate rewrites files instead of checking

app/Makefile

@@ -3,10 +3,12 @@
 MISE_BIN ?= mise
 MISE_EXEC = $(MISE_BIN) exec --
 
-.PHONY: check precommit deps assets compile test test.coverage coverage crap format credo verify
+.PHONY: check precommit deps assets compile test test.coverage coverage crap format format.check credo verify
 
-# Run all static analysis checks (no database required)
-check: credo format
+# Run all static analysis checks (no database required). `format.check` only
+# reports: a gate that rewrites the files it judges can never fail. Fix
+# formatting with `make format`.
+check: credo format.check
 
 # Precommit target for CI/pre-commit hooks
 precommit: check compile
@@ -62,10 +64,18 @@ crap:
 	@test -f cover/default.coverdata || { echo "crap: no exported coverage - run 'make test.coverage' first" >&2; exit 1; }
 	MIX_ENV=test $(MISE_EXEC) mix crap
 
-# Format code
+# Format code. MUTATES the tree; deliberately not part of `check` (nor of the
+# Lens pass), which must never write: a gate that fixes what it judges can never
+# fail, and would dirty the tree a push is about to send. `make check` uses
+# `format.check` below instead.
 format:
 	$(MISE_EXEC) mix format
 
+# Verify formatting without writing. Fails with the offending files, so the fix
+# is explicit: run `make format` (or `make -C app format`).
+format.check:
+	$(MISE_EXEC) mix format --check-formatted
+
 # =============================================================================
 # Playwright e2e browser tests
 # =============================================================================