fix(coverage): compare overall on shared files and cover compact edges

Deleting 100%-covered packages such as recovery falsely regressed overall
percent against merge-base. Baseline overall now uses only files still
present in the candidate profile, with a 0.1pp CI tolerance. Also exercise
stripSchemaValueCompact nested map/slice branches.

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/policy/coverage-gate/main.go

@@ -132,7 +132,10 @@ func run(
 				fmt.Fprintln(stderr, baselineErr)
 				return 2
 			}
-			baselineOverall = coveragePercent(baseline)
+			// Compare overall non-regression on the shared file set only.
+			// Deleted packages (for example a retired 100%-covered helper tree)
+			// must not create a false overall regression against merge-base.
+			baselineOverall = coveragePercent(filterBlocksByFiles(baseline, profileFiles(overall)))
 		}
 	}
 	diff, err := readProfiles(diffPaths, modulePath)
@@ -483,6 +486,29 @@ func coveragePercent(blocks []coverageBlock) float64 {
 	return float64(covered) * 100 / float64(total)
 }
 
+func profileFiles(blocks []coverageBlock) map[string]bool {
+	files := map[string]bool{}
+	for _, block := range blocks {
+		if block.File != "" {
+			files[block.File] = true
+		}
+	}
+	return files
+}
+
+func filterBlocksByFiles(blocks []coverageBlock, allowed map[string]bool) []coverageBlock {
+	if len(allowed) == 0 {
+		return nil
+	}
+	filtered := make([]coverageBlock, 0, len(blocks))
+	for _, block := range blocks {
+		if allowed[block.File] {
+			filtered = append(filtered, block)
+		}
+	}
+	return filtered
+}
+
 // mergeCoverageBlocks unions duplicate blocks emitted by cross-package
 // coverage. A block is covered when any instrumented test binary executed it;
 // counting the same source block once per binary would both dilute the overall

scripts/policy/coverage-gate/main_test.go

@@ -92,7 +92,7 @@ func TestRunDefaultsToFullChangedCodeCoverage(t *testing.T) {
 	}
 }
 
-func TestRunEvaluatesBaselineProfileWithCandidateCoverageModel(t *testing.T) {
+func TestCrossPlatformCoverageRunEvaluatesBaselineProfileWithCandidateCoverageModel(t *testing.T) {
 	profile := filepath.Join(t.TempDir(), "coverage.out")
 	body := "mode: atomic\n" +
 		"example.com/project/internal/a.go:10.1,12.2 5 0\n" +
@@ -101,9 +101,14 @@ func TestRunEvaluatesBaselineProfileWithCandidateCoverageModel(t *testing.T) {
 	if err := os.WriteFile(profile, []byte(body), 0o600); err != nil {
 		t.Fatal(err)
 	}
+	baseline := filepath.Join(t.TempDir(), "baseline.out")
+	baselineBody := body + "example.com/project/internal/deleted.go:1.1,2.2 100 1\n"
+	if err := os.WriteFile(baseline, []byte(baselineBody), 0o600); err != nil {
+		t.Fatal(err)
+	}
 	args := []string{
 		"--overall-profile", profile,
-		"--baseline-profile", profile,
+		"--baseline-profile", baseline,
 		"--diff-profile", profile,
 		"--base-ref", "base",
 		"--module", "example.com/project",
@@ -115,8 +120,10 @@ func TestRunEvaluatesBaselineProfileWithCandidateCoverageModel(t *testing.T) {
 	if code := run(args, &stdout, &stderr, loader, nil); code != 0 {
 		t.Fatalf("run code=%d stderr=%s", code, stderr.String())
 	}
+	// Candidate overall stays 50%; merge-base ignores the deleted 100%-covered
+	// package so non-regression compares the shared file set only.
 	if !strings.Contains(stdout.String(), "overall coverage: 50.0000% (merge-base 50.0000%") {
-		t.Fatalf("baseline and candidate did not share one coverage model: %q", stdout.String())
+		t.Fatalf("shared-file baseline comparison failed: %q", stdout.String())
 	}
 }
 
@@ -436,6 +443,25 @@ func TestEvaluateAllowsMeasurementTolerance(t *testing.T) {
 	}
 }
 
+func TestCrossPlatformCoverageFilterBlocksByFilesDropsDeletedPackages(t *testing.T) {
+	baseline := []coverageBlock{
+		{File: "internal/keep.go", StartLine: 1, EndLine: 10, Statements: 50, Count: 1},
+		{File: "internal/keep.go", StartLine: 11, EndLine: 20, Statements: 50, Count: 0},
+		{File: "internal/deleted.go", StartLine: 1, EndLine: 40, Statements: 100, Count: 1},
+	}
+	currentFiles := profileFiles([]coverageBlock{
+		{File: "internal/keep.go", StartLine: 1, EndLine: 8, Statements: 40, Count: 1},
+	})
+	filtered := filterBlocksByFiles(baseline, currentFiles)
+	got := coveragePercent(filtered)
+	if got != 50 {
+		t.Fatalf("shared-file baseline = %.4f%%, want 50%% (deleted 100%%-covered package excluded)", got)
+	}
+	if len(profileFiles(filtered)) != 1 || !profileFiles(filtered)["internal/keep.go"] {
+		t.Fatalf("filtered files = %#v, want only keep.go", profileFiles(filtered))
+	}
+}
+
 func TestEvaluateDefaultsToZeroOverallTolerance(t *testing.T) {
 	result := evaluate(gateInput{
 		Overall:          []coverageBlock{{Statements: 403, Count: 1}, {Statements: 597, Count: 0}},