feat(0121b): audit_e2e_coverage_go_infra + fn doctor e2e-coverage subcmd

- Crea functions/infra/audit_e2e_coverage.go: AuditE2ECoverage(roots) escanea
  app.md recursivamente, detecta e2e_checks: en frontmatter, retorna
  E2ECoverageReport{total, with_checks, missing, coverage_pct}.
- Crea functions/infra/e2e_coverage_report.go: tipo E2ECoverageReport con
  JSON tags (total, with_checks, missing, coverage_pct).
- Crea types/infra/e2e_coverage_report.md: metadata del tipo para registry.
- Crea functions/infra/audit_e2e_coverage.md: documentacion self-contained
  con Ejemplo, Cuando usarla, Gotchas.
- Crea functions/infra/audit_e2e_coverage_test.go: 3 tests (empty, all-covered,
  partial) — todos pasan.
- Edita cmd/fn/doctor.go: agrega case "e2e-coverage" -> doctorE2ECoverage().
  Output text (tabla tabwriter + lista de apps missing) y --json (E2ECoverageReport).

Acceptance verificado:
  fn doctor e2e-coverage --json -> {total, with_checks, missing, coverage_pct} OK
  fn doctor e2e-coverage        -> tabla text OK
  go test ./functions/infra/... -> 3/3 PASS
  fn show audit_e2e_coverage_go_infra -> indexada OK

task_run: task_d285372493cce2e6 iter 1

Co-authored-by: fn-orquestador <noreply@fn-registry>

cmd/fn/doctor.go

@@ -68,6 +68,8 @@ func cmdDoctor(args []string) {
 		doctorModules(r, jsonOut)
 	case "dod":
 		doctorDod(r, jsonOut)
+	case "e2e-coverage":
+		doctorE2ECoverage(r, jsonOut)
 	default:
 		fmt.Fprintf(os.Stderr, "unknown doctor subcommand: %s\n", sub)
 		doctorUsage()
@@ -97,6 +99,7 @@ Subcommands:
   app-location    Detecta artefactos (apps/analysis) en carpetas de lenguaje (cpp/apps/, etc.) - issue 0096
   modules         Drift entre uses_modules (app.md) y fn_module_<x> link calls (CMakeLists.txt) - issue 0097
   dod             Audita bloque dod_evidence_schema en dev/issues/ y dev/flows/ (issue 0114)
+  e2e-coverage    Porcentaje de apps con e2e_checks declarado en su app.md (issue 0121b)
 
 Flags:
   --json              Salida JSON (para scripting/agentes)
@@ -685,3 +688,41 @@ func doctorDod(root string, jsonOut bool) {
 		}
 	}
 }
+
+func doctorE2ECoverage(root string, jsonOut bool) {
+	roots := []string{
+		filepath.Join(root, "apps"),
+		filepath.Join(root, "cpp", "apps"),
+		filepath.Join(root, "projects"),
+	}
+	report, err := infra.AuditE2ECoverage(roots)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		os.Exit(1)
+	}
+	if jsonOut {
+		emit(report)
+		return
+	}
+
+	w := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
+	fmt.Fprintln(w, "METRIC\tVALUE")
+	fmt.Fprintf(w, "total\t%d\n", report.Total)
+	fmt.Fprintf(w, "with_checks\t%d\n", report.WithChecks)
+	fmt.Fprintf(w, "missing\t%d\n", len(report.Missing))
+	fmt.Fprintf(w, "coverage_pct\t%.2f%%\n", report.CoveragePct)
+	w.Flush()
+
+	if len(report.Missing) > 0 {
+		fmt.Println("\nApps without e2e_checks:")
+		rel := func(p string) string {
+			if r, err := filepath.Rel(root, p); err == nil {
+				return r
+			}
+			return p
+		}
+		for _, m := range report.Missing {
+			fmt.Printf("  %s\n", rel(m))
+		}
+	}
+}