Move internal/log from apex/log to log/slog (closes #77)

internal/log now logs through log/slog. After Init, records go to a small
slog.Handler that prints the same lines as before; before Init they go to
slog.Default(). The helpers and their call sites are unchanged. With
NO_COLOR set on a terminal, log lines are no longer colored.

pterm is dropped: it only printed progress lines, which fmt now writes
directly, outside slog. apex/log, pterm and their indirect dependencies
leave go.mod; golang.org/x/term becomes direct.

simplelog is not imported: importing it replaces slog's default handler in
every program that imports package mfer. The logger stays process-global,
since injecting it would change package mfer's API.

Progress and log lines now share the logger's lock, so the check progress
test runs check ten times to catch a missing wait.

Model: opus-5-5

internal/cli/entry_test.go

@@ -416,6 +416,11 @@ func (w sharedWriter) Write(p []byte) (int, error) {
 // its last progress line and clears it before it logs the summary. Progress
 // writes are slowed down, so a progress goroutine that check did not wait for
 // would write after the summary, or after the run has returned.
+//
+// Progress lines and log lines take the logger's lock, and when the progress
+// goroutine gets it first the clear lands before the summary even without the
+// wait. Which goroutine gets it first changes from run to run, so check runs
+// ten times.
 func TestCheckClearsProgressBeforeSummary(t *testing.T) {
 	t.Parallel()
 
@@ -426,33 +431,35 @@ func TestCheckClearsProgressBeforeSummary(t *testing.T) {
 	opts := testOpts([]string{testApp, cmdGenerate, "-q", "-o", testMF, testDir}, fs)
 	require.Equal(t, 0, runCLI(opts), "generate failed: %s", testStderr(t, opts))
 
-	var (
-		mu     sync.Mutex
-		output bytes.Buffer
-	)
+	for range 10 {
+		var (
+			mu     sync.Mutex
+			output bytes.Buffer
+		)
 
-	opts = testOpts([]string{
-		testApp, cmdCheck, "--progress", testFlagBase, testDir, testMF,
-	}, fs)
-	opts.Stdout = sharedWriter{mu: &mu, buf: &output, delay: 100 * time.Millisecond}
-	opts.Stderr = sharedWriter{mu: &mu, buf: &output}
-	require.Equal(t, 0, runCLI(opts))
+		opts = testOpts([]string{
+			testApp, cmdCheck, "--progress", testFlagBase, testDir, testMF,
+		}, fs)
+		opts.Stdout = sharedWriter{mu: &mu, buf: &output, delay: 100 * time.Millisecond}
+		opts.Stderr = sharedWriter{mu: &mu, buf: &output}
+		require.Equal(t, 0, runCLI(opts))
 
-	mu.Lock()
-	got := output.String()
-	mu.Unlock()
+		mu.Lock()
+		got := output.String()
+		mu.Unlock()
 
-	lastProgress := strings.Index(got, "Checking: 1/1 files")
-	progressDone := strings.Index(got, "\r\033[K")
-	summary := strings.Index(got, "checked 1 files")
+		lastProgress := strings.Index(got, "Checking: 1/1 files")
+		progressDone := strings.Index(got, "\r\033[K")
+		summary := strings.Index(got, "checked 1 files")
 
-	require.NotEqual(t, -1, lastProgress, "no last progress line in %q", got)
-	require.NotEqual(t, -1, progressDone, "progress line never cleared in %q", got)
-	require.NotEqual(t, -1, summary, "no summary in %q", got)
-	assert.Less(t, lastProgress, progressDone,
-		"progress cleared before its last line in %q", got)
-	assert.Less(t, progressDone, summary,
-		"summary logged before progress was cleared in %q", got)
+		require.NotEqual(t, -1, lastProgress, "no last progress line in %q", got)
+		require.NotEqual(t, -1, progressDone, "progress line never cleared in %q", got)
+		require.NotEqual(t, -1, summary, "no summary in %q", got)
+		require.Less(t, lastProgress, progressDone,
+			"progress cleared before its last line in %q", got)
+		require.Less(t, progressDone, summary,
+			"summary logged before progress was cleared in %q", got)
+	}
 }
 
 func TestCheckCommandWithMissingFile(t *testing.T) {